Chimeric T1R taste receptor polypeptides and nucleic acid sequences encoding and cell lines that express said chimeric T1R polypeptides

ABSTRACT

The invention relates to compounds that specifically bind a T1R1/T1R3 or T1R2/T1R3 receptor or fragments or sub-units thereof. The present invention also relates to the use of hetero-oligomeric and chimeric taste receptors comprising T1R1/T1R3 and T1R2/T1R3 in assays to identify compounds that respectively respond to umami taste stimuli and sweet taste stimuli. Further, the invention relates to constitutive cell lines that stably or transiently co-express a combination of T1R1 and T1R3; or T1R2 and T1R3 subunit polypeptides; wherein the T1R1, T1R2 or T1R3 subunit polypeptides are chimeric polypeptides and/or comprise T1R subunits of different species and are expressed in these cell lines under constitutive or inducible conditions. The use of these cells lines in cell-based assays to identify umami and sweet taste modulatory compounds is also provided, particularly high throughput screening assays that detect receptor activity by use of fluorometric imaging.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 10/569,870, filed Feb. 28, 2006, now abandonedwhich is a National Stage of International Application No.PCT/US04/25459, filed Aug. 6, 2004, which claims the benefit of U.S.Provisional Patent Application No. 60/494,071, filed Aug. 6, 2003, andU.S. Provisional Application Ser. No. 60/552,064, filed Mar. 9, 2004,which applications are incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention in part relates to the discovery that the T1Rreceptors assemble to form functional taste receptors. Particularly, ithas been discovered that co-expression of T1R1 and T1R3 results in ataste receptor that responds to umami taste stimuli, includingmonosodium glutamate. Also, it has been discovered that co-expression ofthe T1R2 and T1R3 receptors results in a taste receptor that responds tosweet taste stimuli including naturally occurring and artificialsweeteners.

Also, the present invention relates to the use of hetero-oligomerictaste receptors comprising T1R1/T1R3 and T1R2/T1R3 in assays to identifycompounds that respectively respond to umami taste stimuli and sweettaste stimuli.

The invention also relates to chimeras and truncated versions of T1R1,T1R2, and T1R3, as well as chimeras of T1R1/T1R3 and T1R2/T1R3 receptorscomprising human, rat, or human and rat subunits.

Further, the invention relates to the construction of cell lines thatstably or transiently co-express a combination of T1R1 and T1R3; or T1R2and T1R3, including truncated or chimeric versions of these subunits aswell as chimeric receptors comprising wild-type or chimeric subunits;under constitutive or inducible conditions.

The use of these cell lines in cell-based assays to identify umami andsweet taste modulatory compounds is also provided, particularly highthroughput screening assays that detect receptor activity by the use offluorometric imaging.

The invention also relates to compounds that bind to T1R1/T1R3,T1R2/T1R3 receptors, as well as T1R1, T1R2, and T1R3 chimeric andtruncated subunits and chimeric receptors.

2. Description of the Related Art

The taste system provides sensory information about the chemicalcomposition of the external world. Mammals are believed to have at leastfive basic taste modalities: sweet, bitter, sour, salty, and umami. See,e.g., Kawamura et al., Introduction to Umami: A Basic Taste (1987);Kinnamon et al., Ann. Rev. Physiol., 54:715-31 (1992); Lindemann,Physiol. Rev., 76:718-66 (1996); Stewart et al., Am. J. Physiol.,272:1-26 (1997). Each taste modality is thought to be mediated by adistinct protein receptor or receptors that are expressed in tastereceptor cells found on the surface of the tongue (Lindemann, Physol.Rev. 76:718-716 (1996)). The taste receptors that recognize bitter,sweet, and umami taste stimuli belong to the G-protein-coupled receptor(GPCR) superfamily (Hoon et al., Cell 96:451 (1999); Adler et al., Cell100:693 (2000)). (Other taste modalities are believed to be mediated byion channels.)

G protein-coupled receptors mediate many other physiological functions,such as endocrine function, exocrine function, heart rate, lipolysis,and carbohydrate metabolism. The biochemical analysis and molecularcloning of a number of such receptors has revealed many basic principlesregarding the function of these receptors. For example, U.S. Pat. No.5,691,188 describes how upon a ligand binding to a GPCR, the receptorundergoes a conformational change leading to activation of aheterotrimeric G protein by promoting the displacement of bound GDP byGTP on the surface of the Gα subunit and subsequent dissociation of theGα subunit from the Gβ and Gγ subunits. The free Gα subunits and Gβγcomplexes activate downstream elements of a variety of signaltransduction pathways.

The T1R receptors were previously hypothesized to function as sweettaste receptors (Hoon et al., Cell 96:541-51 (1999); Kitagawa et al.,Biochem Biophys Res. Commun. 283:236-42 (2001); Max et al., Nat. Genet.28:58-63 (2001); Montmayeur et al., Nat. Neurosci. 4: 412-8 (2001);Sainz et al., J. Neurochem. 77: 896-903 (2001)), and Nelson et al.(2001) and Li et al (2002) have recently demonstrated that rat andhuman, respectively, T1R2 and T1R3 act in combination to recognize sweettaste stimuli.

However, there remains in the art a need for new and improved flavoringagents. For example, one of the five known basic tastes is the “savory”or “umami” flavor of monosodium glutamate (“MSG”). MSG is known toproduce adverse reactions in some people, but very little progress hasbeen made in identifying artificial substitutes for MSG. It is knownthat a few naturally occurring materials can increase or enhance theeffectiveness of MSG as a savory flavoring agent, so that less MSG wouldbe needed for a given flavoring application. For example the naturallyoccurring nucleotide compounds inosine monophosphate (IMP) or guanosinemonophosphate (GMP) are known to have a multiplier effect on the savorytaste of MSG, but IMP and GMP are very difficult and expensive toisolate and purify from natural sources, or synthesize, and hence haveonly limited practical application to most commercial needs in food ormedicinal compositions. Less expensive compounds that would provide theflavor of MSG itself, or enhance the effectiveness of any MSG that ispresent could be of very high value. Similarly, discovery of compoundsthat are either new “High Intensity” sweeteners (i.e. they are manytimes sweeter than sucrose) would be of value.

What is needed in the art is the identification and characterization oftaste receptors which function as sweet and umami receptors, assays foridentifying compounds that modulate (enhance or block) sweet and umamitaste, and the compounds that specifically bind to these receptors.

SUMMARY OF THE INVENTION

The present invention provides chimeric receptors comprising variouscombinations of human and rat T1Rs, such as a chimeric T1R2/T1R3receptor comprising a human T1R2 subunit and a rat T1R3 subunit; achimeric T1R2/T1R3 receptor comprising a rat T1R2 subunit and a humanT1R3 subunit; a chimeric T1R2 receptor subunit comprising a humanextracellular domain, a rat transmembrane domain and a rat intracellulardomain; and a chimeric T1R3 receptor subunit comprising a ratextracellular domain, a human transmembrane domain and a humanintracellular domain.

The present invention also provides compounds that specifically bind toT1R1, T1R2, T1R3, T1R1/T1R3 and T1R2/T1R3, or isolated subunits,fragments, chimeras or truncated versions thereof as disclosed herein.

The present invention relates to the discovery that differentcombinations of T1Rs, when co-expressed, produce functional tastereceptors that respond to taste stimuli. Particularly, the presentinvention relates to the discovery that co-expression of T1R2 and T1R3results in a hetero-oligomeric taste receptor that responds to sweettaste stimuli. Also, the present invention relates to the discovery thatthe co-expression of T1R1 and T1R3 results in a hetero-oligomeric tastereceptor that responds to umami taste stimuli such as monosodiumglutamate.

The present invention also relates to cell lines that co-express T1R1and T1R3, including human or rat, or T1R2 and T1R3, including human orrat. In preferred embodiments these cell lines will express elevatedamounts of the receptors, either constitutively or inducibly. These celllines include cells that transiently or stably express T1R1 and T1R3 orT1R2 and T1R3.

Also, the present invention provides assays, preferably high throughputscreening assays, that utilize the T1R2/T1R3 taste receptor, or theT1R1/T1R3 receptor, preferably high throughput cell-based assays, toidentify compounds that modulate sweet or umami taste. The inventionalso provides assays that include taste tests to confirm that thesecompounds modulate sweet or umami taste.

The invention also relates to compounds that bind to the N-terminalextracellular domain of T1R2, compounds that bind to the cysteine-richdomain of T1R2, compounds that bind to the Transmembrane Domain of T1R2,compounds that bind to the Transmembrane Domain of T1R3, compounds thatbind to the Transmembrane Domain of T1R2 of a truncated receptorh2TM/h3TM, and compounds that bind to the Transmembrane Domain of T1R3of a truncated receptor h2TM/h3TM, for example.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 contains a sequence alignment of human and rat T1Rs, humancalcium-sensing receptor and rat metabotropic glutamate receptor.

FIG. 2 shows the coexpression of hT1R2 and hT1R3 in human tongueelpithelium. cDNA specific amplification products are amplified fromcRNA prepared from resected human circumvallate papillae. These RT-PCRamplification experimental results show that hT1R2 and hT1R3 areexpressed in taste tissue.

FIGS. 3A-3B contain data revealing that human T1R2 and T1R3 functiontogether as a sweet taste receptor. These FIGS. contain functional data(intracellular calcium responses) elicited by different sweet tastestimuli in HEK cells stably expressing Galpha 15 that are transientlytransfected with human T1R2, T1R3 and T1R2/T1R3 at variousconcentrations of sweet taste stimuli (FIG. 3A); human T1R2/T1R3 doseresponses for several sweet taste stimuli (FIG. 3B); human T1R2/T1R3responses to sucrose in the presence of gurmarin, and endogenous .beta.2-adrenergic receptor responses to isoproterenol in the presence ofgurmarin. FIG. 3C contains the normalized response to differentsweeteners.

FIG. 4 contains data showing that T1R2 may control T1R2/T1R3 ligandspecificity. The figure contains intracellular calcium responses in HEKcells stably expressing Galpha15, transiently transfected withhT1R2/hT1R3, rT1R2/rT1R3, hT1R2/rT1R3 and rT1R2/hT1R3 in response to 350mM sucrose, 25 mM tryptophan, 15 mM aspartame, and 0.05% monellin.

FIG. 5 contains the results of a fluorescence plate reactor based assaywherein HEK cells stably expressing Gα15 were transiently transfectedwith hT1R2 and hT1R3 or hT1R3 alone and contacted with the calcium dyeFluo-4 and a sweet taste stimulus (12.5 mM cyclamate).

FIG. 6 contains normalized dose-response curves which show that hT1R2and hT1R3 function in combination as the human sweet receptor based ontheir dose-specific interaction with various sweet stimuli (trp,cyclamate, sucrose, neotame, asparame, saccharin and Acek).

FIG. 7 shows that key ligand binding residues of mGluR1 are conserved inT1R1. The figure contains structural information relating to mGluR1 andT1R1 showing the key ligand binding residues which are observed in thesemolecules.

FIGS. 8A-8C show that human T1R1/T1R3 functions as a umami tastereceptor. these FIGS. contain functional data showing HEK cells whichstably express Galpha 15 that are transiently transfected with T1R1/T1R3respond to glutamate in an intracellular calcium-based assay. FIG. 8Ashows that intracellular calcium increases in response to increasingglutamate concentration; FIG. 8B shows intracellular calcium responds toIMP (2 mM), glutamate (0.5 mM) and 0.2 mM IMP; and FIG. 8C shows humanT1R1/T1R3 responses for glutamate in the presence and absence of 0.2 mMIMP.

FIGS. 9A and 9B contain data showing that PDZIP facilitates the surfaceexpression of human T1R2. FIG. 9A shows immunofluorescence staining ofMyc-tagged hT1R2 and indicates that PDZIP significantly increase theamount of human T1R2 protein on the plasma membrane. FIG. 9B containsFACS analysis data demonstrating the same result. More specifically, theFIGS. 9A and 9B respectively contain the results of animmunofluorescence staining assay using Myc-tagged hT1R2 and a FACSexperiment showing that the incorporation of the PDZIP peptide (SEQ IDNo: 1) enhanced the expression of a T1R (hT1R2) on the plasma membrane.

FIGS. 10A through 10L contain calcium imaging data demonstrating thathT1R2/hT1R3 respond to different sweet stimuli.

FIG. 11 shows the responses of cell lines which stably expresshT1R1/hT1R3 by automated fluorescence imaging to umami taste stimuli.

FIG. 12 shows the responses of a cell line which stably expresseshT1R2/hT1R3 by automated fluorescence imaging to sweet taste stimuli.

FIG. 13 shows dose-response curves determined using automatedfluorescence imaging for a cell line that inducibly expresses the humanT1R1/T1R3 taste receptor for L-glutamate in the presence and absence of0.2 mM IMP.

FIGS. 14 and 15 show the response of a cell line that induciblyexpresses the human T1R1/T1R3 taste receptor (I-17 clone) to a panel ofL-amino acids. In FIG. 14 different C-amino acids at 10 mM were testedin the presence and absence of 1 mM IMP. In FIG. 15 dose-responses foractive amino acids were determined in the presence of 0.2 mM IMP.

FIG. 16 shows that lactisole inhibits the receptor activities of humanT1R2/T1R3 and human T1R1/T1R3.

FIG. 17 shows schematics of human-rat T1R chimeras. The chimeras areconstructed by fusing the human or rat extracellular domains to the rator human transmembrane domains respectively, as shown in h2-r2, r2-h2,h3-r3 and r3-h3.

FIG. 18 shows neohesperidin dihydrochalcone (NHDC) enhances theactivities of T1R1/T1R3 umami taste receptor. [Neohesperidindihydrochalcone]=5 μM. The glutamate dose response curve is left-shiftedby 2.3 fold (left panel), and the glutamate/IMP dose response isleft-shifted by 2.1 fold.

FIG. 19 shows that control sweeteners do not affect the activities ofT1R1/T1R3 umami taste receptor [Steviocide]=0.5 mM. [Saccharin]=1 mM.Glutamate dose response is shown in the left panel, and glutamate/IMPdose response is shown in the right panel.

FIG. 20 shows NHDC maps to the transmembrane domain of human T1R3.

FIGS. 21A and 21B show mapping of a compound (sucrose) to the human T1R2transmembrane domain.

FIGS. 22 A-D show sweeteners which map to different domains/subunits ofthe human sweet receptor. FIG. 22A shows responses of human and ratsweet receptors to sucrose (200 mM), aspartame (10 mM), neotame (0.1mM), cyclamate (10 mM), and sucrose (200 mM) in the presence oflactisole (1 mM) (Suc/Lac). HEK-293T cells were transiently transfectedwith human or rat T1R2, T1R3, and a Galpha15 chimera Galpha. 15/il, andassayed for intracellular calcium increases in response to sweeteners.FIG. 22B shows aspartame and neotame were mapped to N-terminalextracellular domain of human T1R2. Combinations of T1R chimeras weretransiently transfected into HEK-293T cells with Galpha. 15/il, andassayed for responses to sweeteners at the concentrations listed in 23A.The presence or absence of response is what is important. FIG. 22C showscyclamate was mapped to the C-termninal transmembrane domain of humanT1R3. FIG. 22D shows lactisole was mapped to the transmembrane domain ofhuman T1R3. Different combinations of T1R chimeras were transientlytransfected into HEK-293T cells with G G.alpha. 15/il, and assayed forresponses to sucrose (200 mM) and AceK (10 mM) in the absence orpresence of lactisole (1 mM). The activities in B, C and D represent themean.+−. SE of number of responding cells for four imaged field of.about. 1,000 confluent cells.

FIGS. 23A-D show mutations in T1R2 or T1R3 selectively affect theactivity of different sweeteners. FIG. 23A shows sequence alignment ofthe N-terminal ligand binding domain of rat mGluR5 with human and rodentTR2s. The 8 critical amino acids involved in ligand-binding in mGluR5are labeled with *, three of the 8 amino acids are conserved in T1R2 andunderlined. FIG. 23B shows two point mutations in the human T1R2N-terminal extracellular domain that abolish response to aspartame andneotame without affecting cyclamate. Stable cell lines of hT1R2/hT1R3(WT), hT1R2 S144A/hT1R3 (S144A) and hT1R2 E302A/hT1R3 (E302A) weregenerated as describe in the Examples. The dose-responses of thesestable lines were determined on FLIPR for sucrose, aspartame, neotameand cyclamate. The activities represent the mean.+−. SE of foldincreases in fluorescence intensities for four recorded wells. FIG. 23Cshows sequence alignment of human and rodent T1R3 transmembrane domains.The three extracellular loops are underlined and labeled EL 1, 2, or 3,according to their order in the protein sequences. FIG. 23D showsmutations in the extracellular loop of hT 1 R 3 that abolish response tocyclamate without affecting aspartame. Each of the three extracellularloops of hT1R3 were replaced with rat protein sequence separately, andthe resulting hT1R3 mutants were transiently transfected into HEK-293Tcells together with G.alpha15/il, and assayed for responses to sucrose(200 mM), aspartame (10 mM) and cyclamate (10 mM). The activitiesrepresent the mean.+−. SE of number of responding cells for four imagedfield of .about. 1,000 confluent cells.

FIGS. 24A-B show human T1R2 is required for Galpha 15-coupling. FIG. 24Ashows responses of human, rat and chimeric sweet receptors to sucrose(200 mM) and AceK (10 mM). Stable Galpha 15 cells were transientlytransfected with human, rat or chimeric T1Rs, and assayed forintracellular calcium increases in response to sweeteners. FIG. 24Bshows G.alpha 15 -coupling is mediated by human T1R2. The activitiesrepresent the mean.+−.SE of number of responding cells for four imagedfield of about 1,000 confluent cells.

FIGS. 25A-F show the effect of lactisole and cyclamate on the humanT1R1/T1R3 umami receptor. FIG. 25A shows the response of human T1R1/T1R3stable cell line to L-glutamate (5 mM) and L-glutamate/IMP (1/0.2 mM) inthe absence and presence of lactisole (5 mM). FIG. 25B shows thelactisole dose-dependent inhibition curves were determined forL-glutamate (Glu), and L-glutamate with 0.2 mM IMP (Glu/IMP), each attwo different concentrations. The IC50s are 0.19.+−.0.02 mM and0.21.+−.0.01 mM for L-glutamate at 8 and 80 mM; 0.35.+−.0.03 mM and0.82.+−.0.06 mM for L-glutamate with IMP at 0.8 and 8 mM respectively.FIG. 25C shows the dose responses for L-glutamate, with or without 0.2mM IMP, were determined in the presence of different concentrations oflactisole. In the presence of 0, 25, or 50 .mu.M lactisole, the EC50sare 9.9.+−.1.5 mM, 7.9.+−.0.5 mM, and 7.0.+−.0.3 mM for L-glutamate; inthe presence of 0, 100, or 200 .mu.M lactisole, the EC50s are0.53.+−.0.04 mM, 0.71.+−.0.10 mM, and 0.84.+−.0.10 mM for L-glutamatewith IMP. Values represent the mean.+−. SE for four independentresponses. FIG. 25D shows the detection thresholds for sweet, umami, andsalty taste stimuli were determined in the presence or absence oflactisole. The inhibition effect of lactisole is shown as fold increasesin detection thresholds. “Detection thresholds” are defined as the lowerlimit of detectable tastants. The detection threshold values wereaveraged over four trials for three subjects. FIG. 25E shows theresponses of human T1R1/T1R3 stable cell line to threshold level ofL-glutamate (4 mM) and endogenous M2 receptor agonist carbachol wereassayed on FLIPR in the absence and presence of various concentrationsof cyclamate. FIG. 25 f shows the dose-responses of the human T1R1/T1R3stable cell line were determined on FLIPR for L-glutamate with orwithout 0.2 mM IMP in the absence and presence of cyclamate (8 mM). Theactivities in B, C, E and F represent the mean.+−. SE of fold increasesin fluorescence intensities for four recorded wells. The dose—responsesin B, C, E and F were reproduced at least 6 times independently.

FIG. 26 shows a working model for the sweet and umami taste receptorstructure-function relationships. Filled arrows indicate directactivation, open arrows indicate enhancement, and bar heads indicateinhibition.

Figure 27A shows all 16 combinations of T1Rs and chimeras that weretested for responses to sweeteners and lactisole. rT1R2/T1R3H-R,rT1R2/hT1R3, and T1R2H-R/T1R3R-H show a significant response tocyclamate and they can be inhibited by lacdisole. T1R chimeras weretransiently transfected into HEK-293T cells with Galpha15/il. Theactivities represent the mean.+−.SE of number of responding cells forfour imaged field of about 1,000 confluent cells, each unit on the Yaxis represents 50 responding cells. Abbreviations: Suc (sucrose 100mM); Suc/Lac (sucrose 100 mM, lactisole 1 mM); AceK (acesulfame K 10mM); AceK/Lac (acesulfame K 10 mM, lactisole 1 mM); ATM (aspartame 10mM); NTM (neotame 10 mM); Cyc (cyclamate 10 mM). FIG. 27B shows thelactisole dose-dependent inhibition curves of the human sweet receptorwere determined for sucrose (Suc), saccharin (Sac), and D-tryptophan(D-Trp), each at two different concentrations. The IC50s are 19.6.+−.0.1.mu.M and 64.6.+−.0.3 .mu.M for sucrose at 50 mM and 120 mM; 22.6.+−.0.1.mu.M and 103.+−.7 .mu.M for saccharin at 0.1 and 2 mM; 19.9.+−.0.2.mu.M and 168.+−.9 .mu.M for D-tryptophan respectively. FIG. 27C showsthe dose responses of human sweet receptor for sucrose, D-Trp andsaccharin were determined with different concentrations of lactisole. Inthe presence of 0, 10, or 20 .mu.M lactisole, the EC50s are 19.4.+−.0.9mM, 24.7.+−.1.0 mM, and 31.3.+−.0.3 mM for sucrose; 0.37.+−.0.02 mM,0.60.+−.0.03 mM, 0.94.+−.0.08 mM for D-Trp; 42.+−.3 .mu.M, 67.+−.6.mu.M, 118.+−.2 .mu.M for saccharin. Values represent the mean.+−.SE forfour independent responses. The dose-responses in B and C weredetermined at least 6 times independently, and generated similar resultsas shown here.

DETAILED DESCRIPTION OF THE INVENTION

The inventions provides compounds that specifically bind to thewild-type and chimeric sweet and umami taste receptors disclosed herein.Further provided are compounds that specifically bind to the wild-type,chimeric or truncated T1R2 or T1R3 subunits of the sweet and umamireceptors.

Binding to the T1R2/T1R3 sweet receptor defines a large genus ofmolecules. The receptor responds to every sweetener tested, includingcarbohydrate sugars, amino acids and derivatives, sweet proteins, andsynthetic sweeteners. In the meantime, the receptor exhibitsstereo-selectivity for certain sweeteners, for example, it responds toD-tryptophan but not L-tryptophan, which is in correlation with tastephysiology data.

Thus, the compounds of the invention specifically bind chimericreceptors. Examples include, but are not limited to, a chimericT1R2/T1R3 receptor comprising a human T1R2 subunit and a rat T1R3subunit; a chimeric T1R2/T1R3 receptor comprising a rat T1R2 subunit anda human T1R3 subunit; a chimeric T1R2 receptor subunit comprising ahuman extracellular domain, a rat transmembrane domain and a ratintracellular domain; and a chimeric T1R3 receptor subunit comprising arat extracellular domain, a human transmembrane domain and a humanintracellular domain. The invention provides functional taste receptors,preferably human taste receptors, that are produced by co-expression ofa combination of different T1Rs, preferably T1R1/T1R3 or T1R2/T1R3, andthe corresponding isolated nucleic acid sequences or fragments,chimeras, or variants thereof that upon co-expression result in afunctional taste receptor, i.e., a sweet taste receptor (T1R2/T1R3) orumami taste receptor (T1R1/T1R3).

T1Rs, a family of class C G protein-coupled receptors (GPCRs), areselectively expressed in the taste tissue (Hoon, M. A., et al., Cell,1999. 96(4): p. 541-51, Bachmanov, A. A., et al., Chem Senses, 2001.26(7): p. 925-33, Montmayeur, J. P., et al., Nat Neurosci, 2001. 4(5):p. 492-8, Max, M., et al., Nat Genet, 2001. 28(1): p. 58-63, Kitagawa,M., et al., Biochem Biophys Res Commun, 2001. 283(1): p. 236-42 andNelson, G., et al., Cell, 2001. 106(3): p. 381-90) Functional expressionof T1Rs in HEK293 cells revealed that different combinations of T1Rsrespond to sweet and umami taste stimuli (Nelson, G., et al., Cell,2001. 106(3): p. 381-90, Li, X., et al., Proc Natl Acad Sci USA, 2002.99(7): p. 4692-6.) T1R2 and T1R3, when co-expressed in 293 cells,recognize diverse natural and synthetic sweeteners [For the reasonmentioned above re “diverse”, please consider whether we need thissection for enablement. If not, I'd delete. We can discuss], while T1R1and T1R3 recognize umami taste stimulus L-glutamate, and this responseis enhanced by 5′-ribonucleotides, a hallmark of umami taste. Knockoutdata confirmed that T1Rs indeed mediate mouse sweet and umami tastes(Damak, S., et al., Science, 2003 301(5634): p. 850-3, Zhao, G. Q., etal., Cell 2003 Oct. 31; 115(3):255-66).

The class C GPCRs possess a large N-terminal extracellular domain, oftenreferred to as the Venus flytrap domain (VFD) (Pin, J. P., PharmacolTher, 2003 98(3): p. 325-54), and are known to function as eitherhomodimers, in the cases of metabotropic glutamate receptors (mGluRs)and calcium-sensing receptor (CaR), or heterodimers, in the case ofγ-aminobutyric acid type B receptor (GABA_(B)R). The functionalexpression data shows a heterodimer mechanism for T1Rs: both T1R1 andT1R2 need to be coexpressed with T1R3 to be functional, which issupported by the overlapping expression patterns of T1Rs in rodenttongue.

It is established herein that T1R family members act in combination withother T1R family members to function as sweet and umami taste receptors.As disclosed in further detail infra in the experimental examples, ithas been demonstrated that heterologous cells which co-express hT1R2 andhT1R3 are selectively activated by sweet taste stimuli in a manner thatmirrors human sweet taste.

For example, HEK-293-Gα15 cells that co-express hT1R2 and hT1R3specifically respond to cyclamate, sucrose, aspartame, and saccharin,and the dose responses for these compounds correlate with thepsychophysical taste detection thresholds.

Also, as supported by data in the experimental examples, it has beenshown that cells which co-express hT1R1 and hT1R3 are selectivelyactivated by glutamate (monosodium glutamate) and 5′-ribonucleotides ina manner that mirrors human umami taste. For example, HEK-293-Gα15 cellsthat co-express hT1R1 and hT1R3 specifically respond to glutamate andthe dose response for this umami-tasting compound correlates with itspsychophysical taste detection threshold. Moreover, 5′-ribonucleotidessuch as IMP enhance the glutamate response of the T1R1/T1R3 receptor, asynergism characteristic of umami taste.

Further, as shown by experimental data in the examples it has been shownthat cells which stably and inducibly co-express T1R1/T1R3 selectivelyrespond to the umami taste stimuli L-glutamate and L-aspartate and onlyweakly respond to other L-amino acids, and at much higherconcentrations, providing further evidence that the T1R1/T1R3 receptorcan be used in assays to identify compounds that modulate (enhance orblock) umami taste stimuli.

Examples of compounds that specifically bind to the sweet receptor andmodulate sweet taste can be found in Table 5.

Tables 1-4 provide examples of compounds that specifically bind to theumami receptor and modulate umami taste.

TABLE 1 Umami Amides Compound Umami Ec₅₀ ratio (vs. @ No. Compound EC₅₀(uM) MSG) (uM) A1

0.22 2.74 1 3,6-Dichloro-N-(4-ethoxy-phenyl)-2- methoxy-benzamide A2

0.93 6.98 0.01 4-(3,6-Dichloro-2-methoxy- benzoylamino)-benzoic acidmethyl ester A3

1.08 6.14 0.03 2,5-dichloro-N-(4- ethoxyphenyl)benzamide A4

0.4 2-[(Benzo[b]thiophene-2-carbonyl)- amino]-4-methyl-pentanoic acidmethyl ester A5

0.31 2-[(Benzofuran-2-carbonyl)-amino]-4- methyl-pentanoic acid methylester A6

0.32 2.86 1 2-[(5-Methoxy-benzofuran-2-carbonyl)-amino]-4-methyl-pentanoic acid methyl ester A7

0.46 (R)-5-methoxy-N-(1-methoxy-4- methylpentan-2-yl)benzofuran-2-carboxamide A8

0.5 5-methyl-N-(5-methylhexan-3-yl) benzofuran-2-carboxamide A9

0.71 2-[(Benzofuran-5-carbonyl)-amino]-4- methyl-pentanoic acid methylester(R)- methyl 2-(benzofuran-5-carboxamido)- 4-methylpentanoate A10

0.91 4.51 1 N-(heptan-4-yl)-5-methoxybenzofuran- 2-carboxamide A11

1.05 6.5 0.3 5-chloro-N-(1-methoxybutan-2- yl)benzofuran-2-carboxamideA12

1.13 5-methoxy-N-(2-methylhexan-3- yl)benzofuran-2-carboxamide A13

1.14 4.46 1 5-methoxy-N-(pentan-3-yl)benzofuran- 2-carboxamide A14

1.14 2-[(5-Methoxy-benzofuran-2- carbonyl)-amino]-4-methylsulfanyl-butyric acid methyl ester methyl 2-(5-methoxybenzofuran-2-carboxamido)-4- (methylthio)butanoate A15

1.14 (1R,2R)-ethyl 2-(5- methoxybenzofuran-2-carboxamido)cyclohexanecarboxylate A16

1.18 5-methoxy-N-(2-methylpentan-3- yl)benzofuran-2-carboxamide A17

1.2 N-(2,4-dimethylpentan-3-yl)-5- methoxybenzofuran-2-carboxamide A18

1.27 5-methoxy-N-(2-methylheptan-4- yl)benzofuran-2-carboxamide A19

1.3 5-methoxy-N-(1-methoxypentan-2- yl)benzofuran-2-carboxamide A20

1.32 5-methyl-N-(2-methylheptan-4-yl) benzofuran-2-carboxamide A21

1.52 3.74 1 N-(pentan-3-yl)benzofuran-2- carboxamide A22

1.58 Benzothiazole-6-carboxylic acid (1- propyl-butyl)-amide A23

0.38 2-methyl-N-(2-methylheptan-4- yl)benzo[d]oxazole-5-carboxamide A24

1.12 2-methyl-N-(2-methylheptan-4- yl)benzo[d]oxazole-6-carboxamide A25

1.48 (R)-4-Methyl-2-[(2-methyl- benzooxazo1e-6-carbonyl)-amino]-pentanoic acid methyl ester A26

1.6 2-methyl-N-(2-methylhexan-3- yl)benzo[d]oxazole-6-carboxamide A27

1.61 2-ethyl-N-(heptan-4-yl)benzo[d] oxazole-6-carboxamide A28

1.69 (R)-4-Methyl-2-[(2-methyl- benzooxazole-5-carbonyl)-amino]-pentanoic acid methyl ester A29

1.91 N-(heptan-4-yl)benzo[d] oxazole-6-carboxamide A30

0.49 12.6 1 5-bromo-N-(heptan-4-yl)furan-2- carboxamide A31

0.62 10.04 1 N-(heptan-4-yl)-4,5-dimethylfuran-2- carboxamide A32

1.15 N-(2,3-dimethylcyclohexyl)-3- methylfuran-2-carboxamide A33

1.33 4,5-dimethyl-N-(2-methylcyclohexyl) furan-2-carboxamide A34

0.53 (R)-methyl 2-(1H-indole-2- carboxamido)-4-methylpentanoate A35

0.82 8.81 1 N-(heptan-4-yl)-1H-indole-6- carboxamide A36

1.01 (R)-methyl 2-(1H-indole-5- carboxamido)-4-methylpentanoate A37

1.5 (R)-methyl 4-methyl-2-(quinblme-6- carboxamido)pentanoate A38

1.22 6.54 1 5-Methyl-thiophene-2-carboxylic acid (1-propyl-butyl)-amideA39

1.31 2.3 1 5-Methyl-thiophene-2-carboxylic acid(1,2,3,4-tetrahydro-naphthalen-1-yl)- amide A40

0.37 (R)-methyl 2-(2-naphthamido)-4- methylpentanoate A41

0.7 2.14 3 N-(nonan-5-yl)benzo[d][1,3]dioxole-5- carboxamide A42

0.35 (2R,3R)-methyl 2- (benzo[d][1,3]dioxole-5-carboxamido)-3-methylpentanoate A43

0.49 2-[(Benzo[1,3]dioxole-5-carbonyl)- amino]-hexanoic acid methylester A44

0.61 (R)-2-[(Benzo[1,3]dioxole-5- carbonyl)-amino]-hexanoic acid methylester A45

0.88 (R)-ethyl 2-(benzo[d][1,3]dioxole-5-carboxamido)-4-methylpentanoate A46

1.32 (R)-methyl 2-(2,3-dihydrobenzofuran-5-carboxamido)-4-methylpentanoate A47

1.33 6.42 0.1 (S)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)benzo[d][1,3]dioxole-5-carboxamide A48

1.51 9.27 1 N-(4-phenylbutan-2-yl)benzo[d] [1,3]dioxole-5-carboxamideA49

1.54 9.53 1 2-[(Benzo[1,3]dioxle-5-carbonyl)- amino]-pentanoic acidmethyl ester A50

1.57 N-(benzo[d][1,3]dioxol-5-yl)-2- propylpentanamide A51

1.58 (R)-propyl 2-(benzo[d][1,3]dioxole-5-carboxamido)-4-methylpentanoate A52

1.65 N-(heptan-4-yl)-2,3- dihydrobenzofuran-5-carboxamide A53

1.83 N-(hexan-3-yl)benzo[d][1,3] dioxole-5-carboxamide A54

0.12 N-(hexan-3-yl)-3-methyl-4- (methylthio)benzamide A55

0.12 methyl 2-(3-chloro-4- methoxybenzamido)hexanoate A56

0.14 N-(hexan-3-yl)-3,4-imethylbenzamide A57

0.18 (R)-methyl 4-methyl-2-(4- vinylbenzamido)pentanoate A58

0.2 4-methoxy-3-methyl-N-(2- methylpentan-3-yl)benzamide A59

0.2 4-methoxy-3-methyl-N-(2- methylhexan-3-yl)benzamide A60

0.2 (R)-methyl 2-(4- (ethylthio)benzamido)-4- methylpentanoate A61

0.22 N-(heptan-4-yl)-4-methoxy-3- methylbenzamide A62

0.25 (R)-methyl 2-(3,4- dimethylbenzamido)-3- methylbutanoate A63

0.25 (R)-methyl 2-(4-methoxy-3- methylbenzamido)-4-methylpentanoate A64

0.26 4-ethoxy-3-methyl-N-(pentan-3- yl)benzamide A65

0.29 (R)-N-(1-methoxy-4-methylpentan-2-yl)-3-methyl-4-(methylthio)benzamide A66

0.29 N-(2,4-dimethoxybenzyl)-3-(1H- pyrrol-1-yl)isonicotinamide A67

0.29 10.75 1 methyl 2-(3-chloro-4- methoxybenzamido)pentanoate A68

0.32 2.62 0.3 4-ethoxy-N-(heptan-4-yl)benzamide A69

0.32 (R)-methyl 4-methyl-2-(4- methylbenzamido)pentanoate A70

0.33 N-(heptan-4-yl)-3- (trifluoromethyl)benzamide A71

0.34 4-ethyl-N-(heptan-4-yl)benzamide A72

0.34 4-ethoxy-3-methyl-N-(5-methylhexan- 3-yl)benzamide A73

0.34 (R)-methyl 2-(3-methoxy-4- methylbenzamido)-4-methylpentanoate A74

0.35 4.98 0.3 3-fluoro-N-(heptan-4-yl)-4- methoxybenzamide A75

0.39 N-(heptan-4-yl)-4- (methylthio)benzamide A76

0.4 4-methoxy-3-methyl-N-(4- phenylbutan-2-yl)benzamide A77

0.44 3-chloro-4-methoxy-N-(2- methylcyclohexyl)benzamide A78

0.46 10.22 0.3 N-(heptan-4-yl)-4-vinylbenzamide A79

0.46 N-(heptan-4-yl)-4-methoxybenzamide A80

0.47 5.12 0.1 3-chloro-4-methoxy-N-(pentan-2- yl)benzamide A81

0.5 N-(hexan-3-yl)-4-methyl-3- (methylthio)benzamide A82

0.51 (R)-methyl 4-methyl-2-(4- propoxybenzamido)pentanoate A83

0.52 N-(heptan-4-yl)-3-methylbenzamide A84

0.53 N-(heptan-4-yl)-2-hydroxy-3- methoxybenzamide A85

0.53 (R)-methyl 2-(3,5- dimethylbenzamido)4- methylpentanoate A86

0.53 methyl 2-(4-methoxy-3- methylbenzamido)-4- (methylthio)butanoateA87

0.54 3.8 1 2-hydroxy-3-methoxy-N-(1,2,3,4-tetrahydronaphthalen-1-yl)benzamide A88

0.55 N-(2,4-dimethylpentan-3-yl)-3-methyl- 4-(methylthio)benzamide A89

0.6 2.85 1 (R)-3-chloro-4-methoxy-N-(1-(4- methoxyphenyl)ethyl)benzamideA90

0.61 N-(heptan-4-yl)-3-methoxybenzamide A91

0.62 (R)-methyl 4-methyl-2-(4- propylbenzamido)pentanoate A92

0.65 4-ethoxy-3-methyl-N-(2-methylheptan- 4-yl)benzamide A93

0.7 5.7 1 (S)-2-hydroxy-3-methoxy-N-(1,2,3,4-tetrahydronaphthalen-1-yl)benzamide A94

0.72 (R)-4-methoxy-N-(2-methoxy-1- phenylethyl)-3-methylbenzamide A95

0.74 (R)-methyl 2-(4-methoxy-3,5- dimethylbenzamido)-4- methylpentanoateA96

0.76 4-methoxy-N-(1-(4- methoxyphenyl)propyl)-3- methylbenzamide A97

0.85 4-methoxy-N-(1-methoxypentan-2-yl)- 3-methylbenzamide A98

0.88 3-chloro-N-(1-hydroxy-4- methylpentan-2-yl)-4- methoxybenzamide A99

0.89 (R)-methyl 4-methyl-2-(3- methylbenzamido)pentanoate A100

1.1 3-chloro-4-methoxy-N-(1-p tolylethyl)benzamide A101

1.16 7.62 1 N-(heptan-4-yl)-2-hydroxy-4- methoxybenzamide A102

1.32 9.49 1 4-hydroxy-3-methyl-N-(1,2,3,4-tetrahydronaphthalen-1-yl)benzamide A103

1.36 (1S,2R)-ethyl 2-(3-chloro-4- methoxybenzamido)cyclohexanecarboxylate A104

1.37 Biphenyl-2-carboxylic acid 2,4- dimethoxy-benzylamide A105

1.38 2.79 1 (S)-N-(1,2,3,4-tetrahydronaphthalen-1- yl)-4-vinylbenzamideA106

1.39 4.01 0.3 3-chloro-N-(2,3-dihydro-1H-inden-1- yl)-4-methoxybenzamide

TABLE 2 Umami Oxalamides Compound Umami EC₅₀ Ec₅₀ ratio (vs. No.Compound (uM) MSG) B1

0.18 N1-(2,4-dimethoxybenzyl)-N2-(2-(furan-2- yl)ethyl)oxalamide B2

0.19 N1-(4-ethoxy-2-methoxybenzyl)-N2-(2-(5-methylpyridin-2-yl)ethyl)oxalamide B3

0.81 N-(3-Methyl-benzo[b]thiophen-2-ylmethyl)-N′-(2-pyridin-2-yl-ethyl)-oxalamide B4

1.22 N1-(2-isopropoxybenzyl)-N2-(2-(pyridin-2- yl)ethyl)oxalamide

TABLE 3 Umami Ureas Umami Ec₅₀ Compound EC₅₀ ratio (vs. Con. No. IUPACName uM MSG) (uM) C1

0.37 4.95 1 1-(2-chlorophenyl)-3-(heptan-4-yl)urea C2

0.49 4.52 1 1-(2,4-dichlorophenyl)-3-(1-phenylpropyl)urea C3

0.52 3.24 3 1-(2-chlorophenyl)-3-(2-methylcyclohexyl)urea C4

0.79 12.15 3 1-(2-fluorophenyl)-3-(heptan-4-yl)urea C5

0.84 9.08 1 1-(2-chlorophenyl)-3-(1-cyclohexylethyl)urea C6

0.98 1-(4-isopropylphenyl)-3-(2-(pyridin-2-yl)ethyl)urea C7

0.99 3.68 1 1-(2-chlorophenyl)-3-(1,2,3,4-tetrahydronaphthalen-1-yl)urea C8

1.41 2.62 0.3 1-(2,4-dimethoxyphenyl)-3-(2- methylcyclohexyl)urea C9

1.42 1-(2-ethylphenyl)-3-(heptan-4-yl)urea C10

1.51 2.1 0.3 1-(4-ethoxyphenyl)-3-(2-methylcyclohexyl)urea C11

1.65 4.49 1 1-(2-fluorophenyl)-3-(1,2,3,4-tetrahydronaphthalen-1-yl)urea C12

1.67 1-(2-methoxyphenyl)-3-(2-methylcyclohexyl)urea C13

1.72 11.87 1 1-(2,4-dimethoxyphenyl)-3-(pentan-3-yl)urea

TABLE 4 Umami Acrylamides Compound Umami EC₅₀ Ec₅₀ ratio @ No. Compound(uM) (vs. MSG) (uM) D1

0.29 3.46 1 (E)-N-(2,4-dimethylpentan-3-yl)-3-(4-methoxyphenyl)acrylamide D2

0.32 (R,E)-methyl 2-(3-(4-methoxyphenyl) acrylamido)-4-methylpentanoateD3

0.63 (E)-methyl 2-(3-(4-methoxyphenyl) acrylamido)hexanoate D4

0.69 9.73 1 N-(1-Methyl-3-phenyl-propyl)-3- thiophen-2-yl-acrylamide D5

0.72 3.48 0.3 (E)-N-(heptan-4-yl)-3-(4- methoxyphenyl)acrylamide D6

0.75 6.3 1 N-(1-Propyl-butyl)-3-thiophen-2-yl- acrylamide D7

0.82 9.62 1 (E)-3-(4-methoxyphenyl)-N- (pentan-3-yl)acrylamide D8

0.94 (R,E)-3-(4-ethoxyphenyl)-N-(1- methoxy-4-methylpentan-2-yl)acrylamide D9

0.98 (Z)-N-(heptan-4-yl)hex-2-enamide D10

1.09 (R,E)-methyl 4-methyl-2-(3-(thiophen- 3-yl)acrylamido)pentanoateD11

1.17 (R)-methyl 2-cinnamamido-4- methylpentanoate D12

1.28 (E)-4-methyl-N-(2-methylcyclohexyl) pent-2-enamide D13

1.31 2.7 0.3 (E)-N-sec-butyl-3-(4- ethoxyphenyl)acrylamide D14

1.43 8.48 1 (E)-N-(1-methoxybutan-2-yl)-3-(4- methoxyphenyl)acrylamideD15

1.54 2.22 0.3 (E)-N-(heptan-4-yl)-3- (thiophen-3-yl)acrylamide D16

1.56 3.13 1 (E)-3-(3,4-dimethoxyphenyl)-N-(4-phenylbutan-2-yl)acrylamide

TABLE 5 Sweet Enhancer Amides Sweet Umami Umami Compound EC₅₀ EC₅₀ EC₅₀No. Compound uM uM ratio E1

0.19 E2

0.65 E3

1.03 E4

1.61 E5

1.61 E6

1.48 E7

1.81 4.04 E8

1.98 E9

2.36 E10

2.44 E11

2.46 E12

2.85 E13

2.91 E14

2.91 E15

3.02 E16

3.04 E17

3.13 E18

3.38 E19

3.57 E20

4.13 E21

4.19 E22

4.52 E23

4.86 E24

6.04 E25

7.79 E26

8.09 E27

0.14 E28

0.21 E29

0.42 E30

0.45 E31

0.49 E32

0.51 E33

0.63 E34

0.71 E35

0.71 E36

0.72 E37

0.77 E38

0.9 E39

0.91 E40

0.95 9.77 E41

1.02 E42

1.06 E43

1.08 E44

1.08 E45

1.13 E46

1.25 E47

1.25 E48

1.29 E49

1.39 E50

1.41 E51

1.49 E52

1.52 E53

1.7 E54

1.83 10.66 E55

1.89 E56

1.92 2.08 E57

1.95 E58

2.23 E59

2.34 2.07 E60

2.37 E61

2.4 E62

2.42 E63

2.6 E64

2.77 E65

2.83 E66

2.86 E67

2.98 E68

3.03 0.33 E69

3.19 E70

3.2 E71

3.33 E72

3.35 E73

3.36 E74

3.62 E75

3.78 E76

3.99 E77

4.11 E78

4.24 8.51 E79

4.28 E80

4.29 E81

4.37 6.98 E82

4.48 E83

4.68 E84

0.83 16.51 E85

1.42 E86

1.48 E87

1.55 E88

1.84 E89

2.56 E90

2.71 E91

2.74 E92

2.74 E93

3.24 E94

3.56 E95

3.58 E96

3.73 E97

4.25 E98

4.63 E99

0.93 E100

1.3 E101

1.37 E102

2.01 E103

2.58 E104

3.05 E105

3.4 E106

3.85 E107

4.15 E108

4.99 E109

2.34 E110

2.5 E111

4.27 E112

4.33 E113

4.44

Also, as supported by experimental data in the examples, it has beenshown that cell lines which co-express T1R1/T1R3 or T1R2/T1R3respectively respond to umami or sweet taste stimuli and a quantitativedose-responsive manner which further supports a conclusion that specificbinding to the T1R1/T1R3 and T1R2/T1R3 receptor can be used to definereceptor agonists and antagonists, e.g., MSG substitutes, umamiblockers, novel artificial and natural sweeteners, and sweet blockers.

Also, as supported by data in experimental examples, it has been shownthat the sweet taste blocker lactisole inhibits both the T1R2/T1R3 sweetreceptor and the T1R1/T1R3 umami taste receptor. Compounds are providedherein that enhance, mimic, modulate or block sweet or umami taste. Thefact that lactisole inhibits both the T1R1/T1R3 and T1R2/T1R3 receptorssuggests that these receptors may share a common subunit which is boundby lactisole and potentially other taste modulators. Therefore, thisshows that some compounds which enhance, mimic, modulate or block sweettaste can have a similar effect on umami taste or vice versa.

Further, as supported by data in experimental examples, it has beendemonstrated that cell lines which stably co-express T1Rs, i.e.T1R1/T1R3 or T1R2/T1R3, when assayed by automated fluorescence imagingvery effectively respond to various sweet and umami taste stimuli, i.e.at magnitudes substantially greater than transiently transfected cells.Thus, these cell lines are especially well suited for use in highthroughput screening assays for identifying compounds that modulate,block, mimic or enhance sweet or umami taste. However, the inventionalso encompasses assays that utilize cells that transiently express aT1R or combination thereof.

Moreover, while the application contains data demonstrating that someT1Rs act in combination, particularly T1R1/T1R3 and T1R2/T1R3, and thatsuch receptor combinations may be used in assays, preferably highthroughput assays, it should be noted that the subject invention alsoencompasses assays that utilize T1R1, T1R2 and T1R3 alone or incombination with other proteins, e.g., other GPCRs.

There are differences in human and rodent sweet taste in terms of theligand specificity, G protein coupling efficiency, as well assensitivity to inhibitors. The species differences in T1R ligandspecificity can be utilized to demonstrate that the sweet taste receptorindeed functions as a heteromeric complex, and that there is more thanone ligand binding site on the receptor. Furthermore, a functional linkbetween the sweet and umami receptors mediated by T1R3 has been shown(Example 16).

Both human and rat sweet receptors can efficiently couple to a chimericGα15 with the C-terminal tail sequence from G_(αil) (G_(α15/il)). Forexample, human but not rat T1R2/T1R3 selectively responds to a group ofsweeteners, including aspartame, neotame, and cyclamate. This isconsistent with taste physiology data. These differences in agonistspecificity can be utilized to map their binding sites on the receptor.A chimeric T1R can be generated between human and rat genes, with ajunction immediately before the transmembrane domain. Each T1R chimeratherefore consists of two halves, the N-terminal extracellular domain,and the C-terminal transmembrane and intracellular domain, fromdifferent species. For example, a chimeric T1R2, termed T1R2-R, has asequence from the N-terminus of human T1R2 fused to rat T1R2C-terminalsequence. Responses to these chimeras can then be tested (FIG. 22).

Novel compounds and novel flavor, tastants, and sweet enhancers werediscovered in the chemistry series of amide derivatives. The amidecompounds also comprise certain sub-classes of amide derivatives orclasses of derivatives related to amides, such as for example ureas,urethanes, oxalamides, acrylamides, and the like. These compounds, whenused together with sucrose or alone, increase a response in vitro andconcomitant increase in sweet perception in human tasting. Thesecompounds enhance other natural and synthetic sweet tastants. Examplesof these compounds are listed in Table 5.

In one embodiment, the invention provides novel compounds, flavorants,tastants, flavor enhancers, taste enhancers, flavor modifying compounds,and/or compositions containing them.

In a more specific embodiment, the invention provides novel sweetflavorants, sweet tastants, sweet taste enhancers, and sweet tastemodifiers and compositions containing them.

More particularly, in another embodiment, the invention is directed tocompounds that modulate, induce, enhance, or inhibit natural orsynthetic sweet tastants, e.g., naturally occurring and syntheticsweeteners.

In another embodiment, the invention provides compositions, preferablycompositions suitable for human or animal consumption, containing atleast one compound of the invention. These compositions include foods,beverages and medicinals, and food additives which when added to foods,beverages or medicinals modulate the flavor or taste thereof,particularly by enhancing the sweet taste thereof.

Another embodiment of the invention is directed to use of a compound ofthe invention to modulate the sweet taste of a desired food, beverage ormedicinal, which composition may comprise one or more other compoundsthat elicit a sweet taste. These compounds, when they were used togetherwith naturally occurring and synthetic sweeteners, not only increased aresponse in vitro but also intensified the sweet and other flavor ortaste perceptions in human tasting. These specific compounds, when theywere used together with sweet tastants, such as naturally occurring andsynthetic sweeteners, not only increased the T1R2/T1R3 response in vitrobut also intensified the sweet taste and other flavor or tasteperceptions in human tasting.

Novel compounds and novel flavor, tastant, and umami enhancers andtastants such as amides, ureas, amino-amides, amido-amides, andβ-lactams are also disclosed herein. These compounds, when used togetherwith MSG or alone, increase a response in vitro and the umami perceptionin human tasting. These compounds also enhance other natural andsynthetic umami tastants. Examples of these compounds are listed inTables 1-4.

In one embodiment, the invention provides novel compounds, flavorants,tastants, flavor enhancers, taste enhancers, flavor modifying compounds,and/or compositions containing them.

In a more specific embodiment, the invention provides novel umamiflavorants, umami tastants, umami taste enhancers, and umami tastemodifiers and compositions containing them.

More particularly, in another embodiment, the invention is directed tocompounds that modulate (induce, enhance or inhibit) natural orsynthetic umami tastants, e.g., monosodium glutamate (MSG).

In another embodiment, the invention provides compositions, preferablycompositions suitable for human or animal consumption, containing atleast one compound of the invention. These compositions include foods,beverages and medicinals, and food additives which when added to foods,beverages or medicinals modulate the flavor or taste thereof,particularly by enhancing the umami taste thereof.

Another embodiment of the invention is directed to use of a compound ofthe invention to modulate the umami taste of a desired food, beverage ormedicinal, which composition may comprise one or more other compoundsthat elicit a umami taste, e.g., MSG. These compounds, when they wereused together with MSG, not only increased a response in vitro but alsointensified the umami and other flavor or taste perceptions in humantasting. These specific compounds, when they were used together withumami tastants, such as MSG, not only increased the T1R1/T1R3 responsein vitro but also intensified the umami taste and other flavor or tasteperceptions in human tasting. Some of the compounds, when they weretasted alone, elicited human perception of umami.

Compounds defined by specific binding to specific receptors using thepresent T1R assays can be used to modulate the taste of foods andbeverages. Suitable assays described in further detail infra include byway of example whole-cell assays and biochemical assays, includingdirect-binding assays using one of a combination of different T1Rreceptors, chimeras or fragments thereof, especially fragmentscontaining N-terminal ligand-binding domains. Examples of assaysappropriate for use in the invention are described in greater detailinfra and are known in the GPCR field.

Assays can be designed that quantitate the binding of differentcompounds or mixtures of compounds to T1R taste receptors or T1R tastereceptor combinations or T1R receptors expressed in combination withother heterologous (non-T1R) proteins, e.g. other GPCRs, or thatquantitate the activation of cells that express T1R taste receptors.This can be effected by stably or transiently expressing taste receptorsin heterologous cells such as HEK-293, CHO and COS cells. Thus, thisphysico-chemical characteristic of the compounds is used to define agenus of compound that share this characteristic.

The assays will preferably use cells that also express (preferablystably) a G protein such as Gα15 or Gα16 or other promiscuous G proteinsor G protein variants, or an endogenous G protein. In addition, G_(β)and G_(γ) proteins may also be expressed therein.

The effect of a compound on sweet or umami taste using cells orcompositions that express or contain the above-identified receptors orreceptor combinations may be determined by various means including theuse of calcium-sensitive dyes, voltage-sensitive dyes, cAMP assays,direct binding assays using fluorescently labeled ligands or radioactiveligands such as ³H-glutamate, or transcriptional assays (using asuitable reporter such as luciferase or beta-lactamase).

Assays that may be utilized with one or more T1Rs according to theinvention include by way of example, assays that utilize a geneticselection for living cells; assays that utilize whole cells or membranefragments or purified T1R proteins; assays that utilize secondmessengers such as cAMP and IP3, assays that detect the translocation ofarrestin to the cell surface, assays that detect the loss of receptorexpression on the cell surface (internalization) by tested ligands,direct ligand-binding assays, competitive-binding assays withinhibitors, assays using in vitro translated protein, assays that detectconformational changes upon the binding of a ligand (e.g., as evidencedby proteolysis, fluorescence, or NMR), behavioral assays that utilizetransgenic non-human animals that express a T1R or T1R combination, suchas flies, worms, or mice, assays that utilize cells infected withrecombinant viruses that contain T1R genes.

Also within the scope of the invention are structure-based analyseswherein the X-ray crystal structure of a T1R or T1R fragment (orcombination of T1Rs, or a combination of a T1R with another protein) isdetermined and utilized to predict by molecular modeling techniquescompounds that will bind to and/or enhance, mimic, block or modulate theparticular T1R receptor or receptor combination. More particularly, theinvention embraces the determination of the crystal structure ofT1R1/T1R3 (preferably hT1R1/hT1R3) and/or T1R2/T1R3 (preferablyhT1R2/hT1R3) and the use of such crystal structures in structure-baseddesign methods to identify molecules that modulate T1R receptoractivity.

The invention especially includes biochemical assays conducted usingcells, e.g., mammalian, yeast, insect or other heterologous cells thatexpress one or more full length T1R receptors or fragments, preferablyN-terminal domains of T1R1, T1R2 and/or T1R3. The effect of a compoundin such assays can be determined using competitive binding assays, e.g.,using radioactive glutamate or IP, fluorescence (e.g., fluorescencepolarization, FRET), or GTPγ ³⁵S binding assays. As noted, in apreferred embodiment, such assays will utilize cell lines that stablyco-express T1R1/T1R3 or T1R2/T1R3 and a suitable G protein, such asG_(α15). Other appropriate G proteins include the chimeric and variant Gproteins disclosed in U.S. application Ser. No. 09/984,292 and60/243,770, incorporated by reference in their entirety herein.

Still further, altered receptors can be constructed and expressed havingimproved properties, e.g., enhanced surface expression or G-proteincoupling. These T1R variants can be incorporated into cell-based andbiochemical assays.

It is envisioned that the present discoveries relating to human T1Rswill extend to other species, e.g., rodents, pigs, monkeys, dogs andcats, and perhaps even non-mammals such as fish. In this regard, severalfish T1R fragments are identified infra in Example 1. Therefore, thesubject invention has application in screening for compounds for use inanimal feed formulations.

The invention further includes that utilize different allelic variantsof various T1Rs and combinations thereof, thereby enabling theidentification of compounds that elicit specific taste sensation inindividuals that express those allelic variants or compounds that elicitspecific taste sensations in all individuals. Such compounds can be usedto make foods more generally palatable.

T1R encoding nucleic acids also provide valuable probes for theidentification of taste cells, as the nucleic acids are specificallyexpressed in taste cells. For example, probes for T1R polypeptides andproteins can be used to identify taste cells present in foliate,circumvallate, and fungiform papillae, as well as taste cells present inthe geschmackstreifen, oral cavity, gastrointestinal epithelium, andepiglottis. In particular, methods of detecting T1Rs can be used toidentify taste cells sensitive to sweet and/or umami taste stimuli orother taste stimuli representing other taste modalities. For example,cells stably or transiently expressing T1R2 and/or T1R3 would bepredicted from the work herein to be responsive to sweet taste stimuli.Similarly, cells expressing T1R1 and/or T1R3 would be predicted to beresponsive to umami taste stimuli. The nucleic acids encoding the T1Rproteins and polypeptides of the invention can be isolated from avariety of sources, genetically engineered, amplified, synthesized,and/or expressed recombinantly according to the methods disclosed in WO00/035374, which is herein incorporated by reference in its entirety. Alisting of T1Rs that may be expressed according to the invention areprovided in the Examples. However, it should be emphasized that theinvention embraces the expression and use of other specific T1Rs orfragments, variants, or chimeras constructed based on such T1Rsequences, and particularly T1Rs of other species.

As disclosed, an important aspect of the invention is the plurality ofmethods of screening for modulators, e.g., activators, inhibitors,stimulators, enhancers, agonists, and antagonists, of thesetaste-cell-specific GPCRs. Such modulators of taste transduction areuseful for the modulation of taste signaling pathways. These methods ofscreening can be used to identify high affinity agonists and antagonistsof taste cell activity. These modulatory compounds can then be used inthe food industry to customize taste, e.g., to modulate the sweet and/orumami tastes of foods.

This invention rectifies the previous lack of understanding relating tosweet and umami taste as it identifies specific T1Rs and T1R receptorcombinations that mediate sweet and umami taste sensation. Therefore, ingeneral, this application relates to the inventors' discoveries relatingto the T1R class of taste-specific G-protein-coupled receptors and theirspecific function in taste perception and the relationship of thesediscoveries to a better understanding of the molecular basis of taste.

The molecular basis of sweet taste and umami taste—the savor ofmonosodium glutamate—is enigmatic. Recently, a three-member class oftaste-specific G-protein-coupled receptors, termed T1Rs, was identified.Overlapping T1R expression patterns and the demonstration that thestructurally related GABA_(B) receptor is heterodimeric suggest that theT1Rs function as heterodimeric taste receptors. In the examples infra,the present inventors describe the functional co-expression of humanT1R1, T1R2, and T1R3 in heterologous cells; cells co-expressing T1R1 andT1R3 are activated by umami taste stimuli; cells co-expressing T1R2 andT1R3 are activated by sweet taste stimuli. T1R1/T1R3 and T1R2/T1R3activity correlated with psychophysical detection thresholds. Inaddition, the 5′-ribonucleotide IMP was found to enhance the T1R1/T1R3response to glutamate, a synergism characteristic of umami taste. Thesefindings demonstrate that specific T1Rs and particularly differentcombinations of the T1Rs function as sweet and umami taste receptors.

Human perception of bitter, sweet, and umami is thought to be mediatedby G-protein-coupled receptors (Lindemann, B., Physiol. Res. 76:718-66(1996)). Recently, evaluation of the human genome revealed the T2R classof bitter taste receptors (Adler et al., Cell 100:613-702 (2000);Chandrasgekar et al., Cell 100:703-11 (2000); Matsunami et al., Nature404: 601-604 (2000)) but the receptors for sweet and umami taste havenot been identified. Recently, another class of candidate tastereceptors, the T1Rs, was identified. The T1Rs were first identified bylarge-scale sequencing of a subtracted cDNA library derived from rattaste tissue, which identified T1R1, and subsequently by T1R1-baseddegenerate PCR, which led to the identification of T1R2 (Hoon et al.,Cell 96:541-551 (1999)). Recently, the present inventors and othersidentified a third and possibly final member of the T1R family, T1R3, inthe human genome databank (Kitagawa et al., Biochem Biophys. Res Commun.283(1): 236-42 (2001); Max et al., Nat. Genet. 28(1): 58-63 (2001);Sainz et al., J. Neurochem. 77(3): 896-903 (2001); Montmayeur et al.,Nat. Neurosci. 4, 492-8. (2001)). Tellingly, mouse T1R3 maps to agenomic interval containing Sac, a locus that influences sweet taste inthe mouse (Fuller et al., J. Hered. 65:33-6 (1974); Li et al., Mamm.Genome 12:13-16 (2001)). Therefore, T1R3 was predicted to function as asweet taste receptor. Recent high-resolution genetic mapping studieshave strengthened the connection between mouse T1R3 and Sac (Fuller T.C., J. Hered. 65(1): 33-36 (1974); Li et al., Mammal. Genome 12(1):13-16 (2001)).

Interestingly, all C-family receptors that have been functionallyexpressed thus far—metabotropic glutamate receptors, the GABA_(B)receptor, the calcium-sensing receptor (Conigrave, A. D., Quinn, S. J. &Brown, E. M., Proc Natl Acad Sci USA 97, 4814-9. (2000)), and a fisholfactory receptor (Speca, D. J. et al., Neuron 23, 487-98. (1999))—havebeen shown to be activated by amino acids. This common feature raisesthe possibility that the T1Rs recognize amino acids, and that the T1Rsmay be involved in the detection of glutamate in addition tosweet-tasting amino acids. Alternatively, a transcriptional variant ofthe mGluR14 metabotropic glutamate receptor has been proposed to be theumami taste receptor because of its selective expression in rat tastetissue, and the similarity of the receptor-activation threshold to theglutamate psychophysical detection threshold (Chaudhari et al., Nat.Neurosci. 3:113-119 (2000)). This hypothesis is difficult to reconcilewith the exceedingly low expression level of the mGluR4 variant in tastetissue, and the more or less unaltered glutamate taste of mGluR4knockout mice (Chaudhari and Roper, Ann. N.Y. Acad. Sci. 855:398-406(1998)). Furthermore, the taste variant is structurally implausible,lacking not only the majority of the residues that form theglutamate-binding pocket of the wild-type receptor, but alsoapproximately half of the globular N-terminal glutamate-binding domain(Kunishima et al., Nature 407:971-7 (2000)).

Comparative analysis of T1R expression patterns in rodents hasdemonstrated that T1R2 and possibly T1R1 are each coexpressed with T1R3(Hoon et al., Cell 96:541-51 (1999); Kitagawa et al., Biochem Biophy.Res. Commun. 283:236-242 (2001); Max et al., Nat. Genet. 28:58-63(2001); Montmayeur et al., Nat. Neurosci 4:492-8 (2001); Sainz et al.,J. Neurochem 77:896-903 (2001)). Furthermore, dimerization is emergingas a common theme of C-family receptors: the metabotropic glutamate andcalcium-sensing receptor are homodimers (Romomano et al., J. Biol. Chem.271:28612-6 (1996); Okamoto et al., J. Biol. Chem. 273: 13089-96 (1998);Han et al., J. Biol. Chem. 274:100008-13 (1999); Bai et al., J. Biol.Chem. 273:23605-10 (1998)), and the structurally related, GABA_(B)receptor is heterodimeric (Jones et al., Nature 396:674-9 (1998);Kaupmann et al., Nature 396:683-687 (1998); White et al., Nature 396:679-682 (1998); Kuner et al., Science 283:74-77 (1999)). The presentinventors have demonstrated by functional coexpression of T1Rs inheterologous cells that human T1R2 functions in combination with humanT1R3 as a sweet taste receptor and that human T1R1 functions incombination with human T1R3 as an umami taste receptor.

The discoveries discussed herein are especially significant, aspreviously the development of improved artificial sweeteners has beenhampered by the lack of assays for sweet taste. Indeed, the fivecommonly used commercial artificial sweeteners, all of which activatehT1R2/hT1R3, were discovered serendipitously. Similarly, other thansensory testing, a laborious process, there is no assay for identifyingcompounds that modulate umami taste. These problems are now alleviatedbecause, as established by experimental results discussed infra, thehuman sweet and umami receptors have been identified, and assays forthese receptors have been developed, particularly assays that use cellsthat stably express a functional T1R taste receptor, i.e. the sweet orumami taste receptor.

Based thereon the invention provides assays for detecting andcharacterizing taste-modulating compounds, wherein T1R family membersact, as they do in the taste bud, as reporter molecules for the effecton sweet and umami taste of taste-modulating compounds. Particularlyprovided and within the scope of the invention are assays foridentifying compounds that modulate, mimic, enhance and/or blockindividually, sweet and umami tastes. Methods for assaying the activityof GPCRs, and especially compounds that affect GPCR activity are wellknown and are applicable to the T1R family member of the presentinvention and functional combinations thereof. Suitable assays have beenidentified supra.

The invention also provides compounds that bind T1R1, T1R2, T1R3,T1R2/T1R3 or T1R1/T1R3, or any fragment, portion, or subunit thereof, asdisclosed throughout.

In particular, the subject GPCRs can be used in assays to, e.g., measurechanges in ligand binding, ion concentration, membrane potential,current flow, ion flux, transcription, receptor-ligand interactions,second messenger concentrations, in vitro and in vivo. In anotherembodiment, T1R family members may be recombinantly expressed in cells,and the modulation of taste transduction via GPCR activity may beassayed by measuring changes in Ca²⁺ levels and other intracellularmessages such as cAMP, cGMP, or IP₃.

In certain assays, a domain of a T1R polypeptide, e.g., anextracellular, transmembrane, or intracellular domain, is fused to aheterologous polypeptide, thereby forming a chimeric polypeptide, e.g.,a chimeric protein with GPCR activity. Particularly contemplated is theuse of fragments of T1R, T1R2 or T1R3 containing the N-terminalligand-binding domain. Such proteins are useful, e.g., in assays toidentify ligands, agonists, antagonists, or other modulators of T1Rreceptors. For example, a T1R polypeptide can be expressed in aeukaryotic cell as a chimeric receptor with a heterologous, chaperonesequence that facilitates plasma membrane trafficking, or maturation andtargeting through the secretory pathway. The optional heterologoussequence may be a PDZ domain-interacting peptide, such as a C-terminalPDZIP fragment (SEQ ID NO 1). PDZIP is an ER export signal, which,according to the present invention, has been shown to facilitate surfaceexpression of heterologous proteins such as the T1R receptors describedherein. More particularly, in one aspect of the invention, PDZIP can beused to promote proper targeting of problematic membrane proteins suchas olfactory receptors, T2R taste receptors, and the T1R taste receptorsdescribed herein.

Examples of such chimeric receptors include trans-species receptors. Anycombination of receptor subunits from various species can be usedtogether to form a chimeric receptor, which can then, be used toidentify tastants, for example. Therefore, contemplated herein is achimeric T1R2/T1R3 receptor comprising a human T1R2 subunit and a ratT1R3 subunit. Also contemplated is a chimeric T1R2/T1R3 receptorcomprising, a rat T1R2 subunit and a human T1R3 subunit. Alsocontemplated is a chimeric T1R2 receptor subunit comprising, a humanextracellular domain, a rat transmembrane domain and a rat intracellulardomain (SEQ ID NOS: 16 and 17, for example). Also contemplated ischimeric T1R3 receptor subunit comprising, a rat extracellular domain, ahuman transmembrane domain and a human intracellular domain (SEQ ID NOS:18 and 19, for example.)

Such chimeric T1R receptors can be expressed in any eukaryotic cell,such as HEK-293 cells. Preferably, the cells contain a G protein,preferably a promiscuous G protein such as G_(α15) or G_(α16) or anothertype of promiscuous G protein capable of linking a wide range of GPCRsto an intracellular signaling pathway or to a signaling protein such asphospholipase C. Activation of such chimeric receptors in such cells canbe detected using any standard method, such as by detecting changes inintracellular calcium by detecting FURA-2 dependent fluorescence in thecell. If preferred host cells do not express an appropriate G protein,they may be transfected with a gene encoding a promiscuous G proteinsuch as those described in U.S. Application Ser. No. 60/243,770, U.S.application Ser. No. 09/984,297, filed Oct. 29, 2001, and U.S.application Ser. No. 09/989,497 filed Nov. 21, 2001 which are hereinincorporated by reference in its entirety.

Additional methods of assaying for modulators of taste transductioninclude in vitro ligand-binding assays using: T1R polypeptides, portionsthereof, i.e., the extracellular domain, transmembrane region, orcombinations thereof, or chimeric proteins comprising one or moredomains of a T1R family member; oocyte or tissue culture cellsexpressing T1R polypeptides, fragments, or fusion proteins;phosphorylation and dephosphorylation of T1R family members; G proteinbinding to GPCRs; ligand-binding assays; voltage, membrane potential andconductance changes; ion flux assays; changes in intracellular secondmessengers such as cGMP, cAMP and inositol triphosphate (IP3); andchanges in intracellular calcium levels.

Further, the invention provides methods of detecting T1R nucleic acidand protein expression, allowing investigation of taste transductionregulation and specific identification of taste receptor cells. T1Rfamily members also provide useful nucleic acid probes for paternity andforensic investigations. T1R genes are also useful as nucleic acidprobes for identifying taste receptor cells, such as foliate, fungiform,circumvallate, geschmackstreifen, and epiglottis taste receptor cells.T1R receptors can also be used to generate monoclonal and polyclonalantibodies useful for identifying taste receptor cells.

Functionally, the T1R polypeptides comprise a family of related seventransmembrane G protein-coupled receptors, which are believed to beinvolved in taste transduction and may interact with a G protein tomediate taste signal transduction (see, e.g., Fong, Cell Signal, 8:217(1996); Baldwin, Curr. Opin. Cell Biol., 6:180 (1994)). Structurally,the nucleotide sequences of T1R family members encode relatedpolypeptides comprising an extracellular domain, seven transmembranedomains, and a cytoplasmic domain. Related T1R family genes from otherspecies share at least about 50%, and optionally 60%, 70%, 80%, or 90%,nucleotide sequence identity over a region of at least about 50nucleotides in length, optionally 100, 200, 500, or more nucleotides inlength to the T1R nucleic acid sequences disclosed herein in theExamples, or conservatively modified variants thereof, or encodepolypeptides sharing at least about 35 to 50%, and optionally 60%, 70%,80%, or 90%, amino acid sequence identity over an amino acid region atleast about 25 amino acids in length, optionally 50 to 100 amino acidsin length to a T1R polypeptide sequence disclosed infra in the Examplesconservatively modified variants thereof.

Several consensus amino acid sequences or domains have also beenidentified that are characteristic of T1R family members. For example,T1R family members typically comprise a sequence having at least about50%, optionally 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95-99%, orhigher, identity to T1R consensus sequences 1 and 2 (SEQ ID NOs. 2 and3, respectively). These conserved domains thus can be used to identifymembers of the T1R family, by identity, specific hybridization oramplification, or specific binding by antibodies raised against adomain. T1R consensus sequences include by way of example the followingsequences:

-   T1R Family Consensus Sequence 1: (SEQ ID NO: 2)-   (TR)C(FL)(RQP)R(RT)(SPV)(VERKT)FL(AE)(WL)(RHG)E-   T1R Family Consensus Sequence 2: (SEQ ID NO: 3)-   (LQ)P(EGT)(NRC)YN(RE)A(RK)(CGF)(VLI)T(FL)(AS)(ML)

These consensus sequences are inclusive of those found in the T1Rpolypeptides described herein, but T1R family members from otherorganisms may be expected to comprise consensus sequences having about75% identity or more to the inclusive consensus sequences describedspecifically herein.

Specific regions of the T1R nucleotide and amino acid sequences may beused to identify polymorphic variants, interspecies homologs, andalleles of T1R family members. This identification can be made in vitro,e.g., under stringent hybridization conditions or PCR (e.g., usingprimers encoding the T1R consensus sequences identified above), or byusing the sequence information in a computer system for comparison withother nucleotide sequences. Different alleles of T1R genes within asingle species population will also be useful in determining whetherdifferences in allelic sequences control differences in taste perceptionbetween members of the population. Classical PCR-type amplification andcloning techniques are useful for isolating new T1Rs, for example, wheredegenerate primers are sufficient for detecting related genes acrossspecies.

Typically, identification of polymorphic variants and alleles of T1Rfamily members can be made by comparing an amino acid sequence of about25 amino acids or more, e.g., 50-100 amino acids. Amino acid identity ofapproximately at least 35 to 50%, and optionally 60%, 70%, 75%, 80%,85%, 90%, 95-99%, or above typically demonstrates that a protein is apolymorphic variant, interspecies homolog, or allele of a T1R familymember. Sequence comparison can be performed using any of the sequencecomparison algorithms discussed below. Antibodies that bind specificallyto T1R polypeptides or a conserved region thereof can also be used toidentify alleles, interspecies homologs, and polymorphic variants.

Polymorphic variants, interspecies homologs, and alleles of T1R genescan be confirmed by examining taste-cell-specific expression of theputative T1R gene or protein. Typically, T1R polypeptides having anamino acid sequence disclosed herein can be used as a positive controlin comparison to the putative T1R polypeptide to demonstrate theidentification of a polymorphic variant or allele of the T1R familymember. The polymorphic variants, alleles, and interspecies homologs areexpected to retain the seven transmembrane structure of a Gprotein-coupled receptor. For further detail, see WO 00/06592, whichdiscloses related T1R family members, GPCR-B3s, the contents of whichare herein incorporated by reference in a manner consistent with thisdisclosure. GPCR-B3 receptors are referred to herein as rT1R1 and mT1R1.Additionally, see WO 00/06593, which also discloses related T1R familymembers, GPCR-B4s, the contents of which are herein incorporated byreference in a manner consistent with this disclosure. GPCR-B4 receptorsare referred to herein as rT1R2 and mT1R2. As discussed previously, theinvention also includes structure-based assays that utilize the x-raycrystalline structure of a T1R or T1R combination, e.g., hT1R2/hT1R3 orhT1R1/hT1R3, to identify molecules that modulate T1R receptor activity,and thereby modulate sweet and/or umami taste.

The present invention also provides assays, preferably high throughputassays, to identify molecules that enhance, mimic; block and/or modulateT1R receptors. In some assays, a particular domain of a T1R familymember is used in combination with a particular domain of another T1Rfamily member, e.g., an extracellular, transmembrane, or intracellulardomain or region. In other embodiments, an extracellular domain,transmembrane region or combination thereof may be bound to a solidsubstrate, and used, e.g., to isolate ligands, agonists, antagonists, orany other molecules that can bind to and/or modulate the activity of aT1R polypeptide.

Various conservative mutations and substitutions are envisioned to bewithin the scope of the invention. For instance, it is within the levelof skill in the art to perform amino acid substitutions using knownprotocols of recombinant gene technology including PCR, gene cloning,site-directed mutagenesis of cDNA, transfection of host cells, andin-vitro transcription. The variants could then be screened foractivity.

DEFINITIONS

As used herein, the following terms have the meanings ascribed to themunless specified otherwise.

“Taste cells” include neuroepithelial cells that are organized intogroups to form taste buds of the tongue, e.g. foliate, fungiform, andcircumvallate cells (see, e.g., Roper et al., Ann. Rev. Neurosci.12:329-353 (1989)). Taste cells are also found in the palate and othertissues, such as the esophagus and the stomach.

“T1R” refers to one or more members of a family of G protein-coupledreceptors that are expressed in taste cells such as foliate, fungiform,and circumvallate cells, as well as cells of the palate, and esophagus(see, e.g., Hoon et al., Cell, 96:541-551 (1999), herein incorporated byreference in its entirety). Members of this family are also referred toas GPCR-B3 and TR1 in WO 00/06592 as well as GPCR-B4 and TR2 in WO00/06593. GPCR-B3 is also herein referred to as rT1R1, and GPCR-B4 isreferred to as rT1R2. Taste receptor cells can also be identified on thebasis of morphology (see, e.g., Roper, supra), or by the expression ofproteins specifically expressed in taste cells. T1R family members mayhave the ability to act as receptors for sweet taste transduction, or todistinguish between various other taste modalities. Representative T1Rsequences, including hT1R1, hT1R2 and hT1R3 are identified infra in theexamples.

“T1R” nucleic acids encode a family of GPCRs with seven transmembraneregions that have “G protein-coupled receptor activity,” e.g., they maybind to G proteins in response to extracellular stimuli and promoteproduction of second messengers such as IP3, cAMP, cGMP, and Ca²⁺ viastimulation of enzymes such as phospholipase C and adenylate cyclase(for a description of the structure and function of GPCRs, see, e.g.,Fong, supra, and Baldwin, supra). A single taste cell may contain manydistinct T1R polypeptides.

The term “T1R” family therefore refers to polymorphic variants, alleles,mutants, and interspecies homologs that: (1) have at least about 35 to50% amino acid sequence identity, optionally about 60, 75, 80, 85, 90,95, 96, 97, 98, or 99% amino acid sequence identity to a T1Rpolypeptide, preferably those identified in Example 1, over a window ofabout 25 amino acids, optionally 50-100 amino acids; (2) specificallybind to antibodies raised against an immunogen comprising an amino acidsequence preferably selected from the group consisting of the T1Rpolypeptide sequence disclosed in Example 1 and conservatively modifiedvariants thereof; (3) are encoded by a nucleic acid molecule whichspecifically hybridize (with a size of at least about 100, optionally atleast about 500-1000 nucleotides) under stringent hybridizationconditions to a sequence selected from the group consisting of the T1Rnucleic acid sequences contained in Example 1, and conservativelymodified variants thereof; or (4) comprise a sequence at least about 35to 50% identical to an amino acid sequence selected from the groupconsisting of the T1R amino acid sequence identified in Example 1.

Topologically, the T1Rs disclosed herein have an “N-terminal domain”also called “extracellular domain” comprising a “venus flytrap domain”and a “cysteine rich domain;” “transmembrane domains” comprising seventransmembrane regions, and corresponding cytoplasmic, and extracellularloops; and a “C-terminal domain” (see, e.g., Hoon et al., Cell,96:541-551 (1999); Buck & Axel, Cell, 65:175-187 (1991)). These domainshave been structurally identified using methods known to those of skillin the art, such as sequence analysis programs that identify hydrophobicand hydrophilic domains (Stryer, Biochemistry, (3rd ed. 1988). Suchdomains are useful for making chimeric proteins and for in vitro assaysof the invention, e.g., ligand binding assays. The specific binding of acompound to these structurally defined domains provides providesstructural definition for the compound.

“Extracellular domains” therefore refers to the domains of T1Rpolypeptides that protrude from the cellular membrane and are exposed tothe extracellular face of the cell. Such domains generally include the“N terminal domain” that is exposed to the extracellular face of thecell, and optionally can include portions of the extracellular loops ofthe transmembrane domain that are exposed to the extracellular face ofthe cell, i.e., the loops between transmembrane regions 2 and 3, betweentransmembrane regions 4 and 5, and between transmembrane regions 6 and7.

The “N-terminal domain” region starts at the N-terminus and extends to aregion close to the start of the first transmembrane domain. Moreparticularly, in one embodiment of the invention, this domain starts atthe N-terminus and ends approximately at the conserved glutamic acid atamino acid position 563 plus or minus approximately 20 amino acids.These extracellular domains are useful for in vitro ligand-bindingassays, both soluble and solid phase. In addition, transmembraneregions, described below, can also bind ligand either in combinationwith the extracellular domain, and are therefore also useful for invitro ligand-binding assays.

“Cysteine-rich domain” refers to the domain of the polypeptides. Thisconserved sequence contains several highly-conserved Cys residues thatform disulphide bridges, and lies outside the cell membrane. This regioncorresponds to the domain of the T1R family members and is found in allthree subunits, T1R1-T1R3. The cysteine rich sequence is found in aminoacids 510-566 of T1R1, 508-565 of T1R2, and 512-568 or T1R3.

“Transmembrane domain,” which comprises the seven “transmembraneregions,” refers to the domain of T1R polypeptides that lies within theplasma membrane, and may also include the corresponding cytoplasmic(intracellular) and extracellular loops. In one embodiment, this regioncorresponds to the domain of T1R family members which startsapproximately at the conserved glutamic acid residue at amino acidposition 563 plus or minus 20 amino acids and ends approximately at theconserved tyrosine amino acid residue at position 812 plus or minusapproximately 10 amino acids. The seven transmembrane regions andextracellular and cytoplasmic loops can be identified using standardmethods, as described in Kyte & Doolittle, J. Mol. Biol., 157:105-32(1982)), or in Stryer, supra.

“Cytoplasmic domains” refers to the domains of T1R polypeptides thatface the inside of the cell, e.g., the “C-terminal domain” and theintracellular loops of the transmembrane domain, e.g., the intracellularloop between transmembrane regions 1 and 2, the intracellular loopbetween transmembrane regions 3 and 4, and the intracellular loopbetween transmembrane regions 5 and 6.

“C-terminal domain” refers to the region that spans the end of the lasttransmembrane domain and the C-terminus of the protein, and which isnormally located within the cytoplasm. In one embodiment, this regionstarts at the conserved tyrosine amino acid residue at position 812 plusor minus approximately 10 amino acids and continues to the C-terminus ofthe polypeptide.

The term “ligand-binding region” or “ligand-binding domain” refers tosequences derived from a taste receptor, particularly a taste receptorthat substantially incorporates at least the extracellular domain of thereceptor. In one embodiment, the extracellular domain of theligand-binding region may include the N-terminal domain and, optionally,portions of the transmembrane domain, such as the extracellular loops ofthe transmembrane domain. The ligand-binding region may be capable ofbinding a ligand, and more particularly, a compound that enhances,mimics, blocks, and/or modulates taste, e.g., sweet or umami taste.

The phrase “heteromultimer” or “heteromultimeric complex” in the contextof the T1R receptors or polypeptides of the invention refers to afunctional association of at least one T1R receptor and anotherreceptor, typically another T1R receptor polypeptide (or, alternativelyanother non-T1R receptor polypeptide). For clarity, the functionalco-dependence of the T1Rs is described in this application as reflectingtheir possible function as heterodimeric taste receptor complexes.However, as discussed previously, functional co-dependence mayalternatively reflect an indirect interaction. For example, T1R3 mayfunction solely to facilitate surface expression of T1R1 and T1R2, whichmay act independently as taste receptors. Alternatively, a functionaltaste receptor may be comprised solely of T1R3, which is differentiallyprocessed under the control of T1R1 or T1R2, analogous to RAMP-dependentprocessing of the calcium-related receptor.

The phrase “functional effects” in the context of assays for testingcompounds that modulate T1R family member mediated taste transductionincludes the determination of any parameter that is indirectly ordirectly under the influence of the receptor, e.g., functional, physicaland chemical effects. It includes ligand binding, changes in ion flux,membrane potential, current flow, transcription, G protein binding, GPCRphosphorylation or dephosphorylation, conformation change-based assays,signal transduction, receptor-ligand interactions, second messengerconcentrations (e.g., cAMP, cGMP, IP3, or intracellular, Ca²⁺), invitro, in vivo, and ex vivo and also includes other physiologic effectssuch increases or decreases of neurotransmitter or hormone release.

By “determining the functional effect” in the context of assays is meantassays for a compound that increases or decreases a parameter that isindirectly or directly under the influence of a T1R family member, e.g.,functional, physical and chemical effects. Such functional effects canbe measured by any means known to those skilled in the art, e.g.,changes in spectroscopic characteristics (e.g., fluorescence,absorbency, refractive index), hydrodynamic (e.g., shape),chromatographic, or solubility properties, patch clamping,voltage-sensitive dyes, whole cell currents, radioisotope efflux,inducible markers, oocyte T1R gene expression; tissue culture cell T1Rexpression; transcriptional activation of T1R genes; ligand-bindingassays; voltage, membrane potential and conductance changes; ion fluxassays; changes in intracellular second messengers such as cAMP, cGMP,and inositol triphosphate (IP3); changes in intracellular calciumlevels; neurotransmitter release, conformational assays and the like.

A “flavor or tastant” herein refers to a compound or biologicallyacceptable salt thereof that induces, in a subject, the perception ofsmell and/or taste, which include sweet, sour, salty, bitter and umami,and others. The subject can be human, animals, and/or a biologicalassay, such as the ones described and cited in this application.

A “flavor or taste modifier” herein refers to a compound or biologicallyacceptable salt thereof that modulates, including enhancing orpotentiating, inhibiting, and inducing, the smell and/or tastes of anatural or synthetic tastants in a subject.

A “flavor or taste enhancer” herein refers to a compound or biologicallyacceptable salt thereof that enhances the tastes or smell of a naturalor synthetic tastants, e.g., monosodium glutamate (MSG) for umami tasteand fructose for sweet taste.

“Umami tastant” or “umami compound” herein refers to a compound orbiologically acceptable salt thereof that elicits a detectable umamitaste in a subject, e.g., MSG.

“Sweet tastant” or “sweet compound” herein refers to a compound orbiologically acceptable salt thereof that elicits a detectable sweettaste in a subject, e.g., fructose.

An “umami taste modifier” herein refers to a compound or biologicallyacceptable salt thereof that modulates, including enhancing orpotentiating, inhibiting, and inducing, the umami taste of a natural orsynthetic umami tastants, e.g., monosodium glutamate (MSG) in a subject.

A “sweet taste modifier” herein refers to a compound or biologicallyacceptable salt thereof that modulates, including enhancing orpotentiating, inhibiting, and inducing, the sweet taste of a natural orsynthetic sweet tastants, e.g., fructose, in a subject.

A “taste enhancing amount” herein refers to an amount of a compound thatis sufficient to enhance the taste of a natural or synthetic tastants,e.g., monosodium glutamate (MSG) for umami taste or fructose for sweettaste.

“Wet Soup Category” means wet/liquid soups regardless of concentrationor container, including frozen Soups. For the purpose of this definitionsoup(s) means a food prepared from meat, poultry, fish, vegetables,grains, fruit and other ingredients, cooked in a liquid which mayinclude visible pieces of some or all of these ingredients. It may beclear (as a broth) or thick (as a chowder), smooth, pureed or chunky,ready-to-serve, semi-condensed or condensed and may be served hot orcold, as a first course or as the main course of a meal or as a betweenmeal snack (sipped like a beverage). Soup may be used as an ingredientfor preparing other meal components and may range from broths (consommé)to sauces (cream or cheese-based soups).

“Dehydrated and Culinary Food Category” means: (i) Cooking aid productssuch as: powders, granules, pastes, concentrated liquid products,including concentrated bouillon, bouillon and bouillon like products inpressed cubes, tablets or powder or granulated form, which are soldseparately as a finished product or as an ingredient within a product,sauces and recipe mixes (regardless of technology); (ii) Meal solutionsproducts such as: dehydrated and freeze dried soups, includingdehydrated soup mixes, dehydrated instant soups, dehydratedready-to-cook soups, dehydrated or ambient preparations of ready-madedishes, meals and single serve entrees including pasta, potato and ricedishes; and (iii) Meal embellishment products such as: condiments,marinades, salad dressings, salad toppings, dips, breading, battermixes, shelf stable spreads, barbecue sauces, liquid recipe mixes,concentrates, sauces or sauce mixes, including recipe mixes for salad,sold as a finished product or as an ingredient within a product, whetherdehydrated, liquid or frozen.

“Beverage Category” means beverages, beverage mixes and concentrates,including but not limited to, alcoholic and non-alcoholic ready to drinkand dry powdered Other examples of foods and beverages wherein compoundsaccording to the invention may be incorporated included by way ofexample carbonated and non-carbonated beverages, e.g., sodas, juices,alcoholic and non-alcoholic beverages, confectionery products, e.g.,cakes, cookies, pies, candies, chewing gums, gelatins, ice creams,sorbets, puddings, jams, jellies, salad dressings, and other condiments,cereal, and other breakfast foods, canned fruits and fruit sauces andthe like.

Additionally, the subject compounds can be used in flavor preparationsto be added to foods and beverages. In preferred instances thecomposition will comprise another flavor or taste modifier such as asweet tastant.

In some instances biologically acceptable salts of the subject compoundsmay be used. Examples of such salts include alkali and earth metalsalts, organic salts, and the like. Specific examples include potassium,sodium, calcium and magnesium salts, hydrochloric or sulfuric acidsalts, ethanolamine salts, and the like. The salt will be selected suchthat it is biologically, safe for ingestion and does adversely affectthe sweet taste modulatory properties of the compound.

As used herein, the term “medicinal product” includes both solids andliquids which are ingestible non-toxic materials which have medicinalvalue such as cough syrups, cough drops, aspirin and chewable medicinaltablets. An oral hygiene product includes solids and liquids such astoothpaste or mouthwash.

A “comestibly or medicinally acceptable carrier or excipient” is amedium that is used to prepare a desired dosage form of the inventivecompound. A comestibly or medicinally acceptable carrier includessolvents, diluents, or other liquid vehicle; dispersion or suspensionaids; surface active agents; isotonic agents; thickening or emulsifyingagents, preservatives; solid binders; lubricants and the like.

“Inhibitors,” “activators,” “enhancers” and “modulators” of T1R genes orproteins are used to refer to inhibitory, activating, enhancing ormodulating molecules identified using in vitro and in vivo assays fortaste transduction, e.g., ligands, agonists, antagonists, and theirhomologs and mimetics.

Inhibitors are compounds that, e.g., bind to, partially or totally blockstimulation, decrease, prevent, delay activation, inactivate,desensitize, or down regulate taste transduction, e.g., antagonists.Activators and enhancers are compounds that, e.g., bind to, enhance,stimulate, increase, open, activate, facilitate, enhance activation,sensitize, or up regulate taste transduction, e.g., agonists. Modulatorsinclude compounds that, e.g., alter the interaction of a receptor with:extracellular proteins that bind activators or inhibitor (e.g., ebnerinand other members of the hydrophobic carrier family); G proteins;kinases (e.g., homologs of rhodopsin kinase and beta adrenergic receptorkinases that are involved in deactivation and desensitization of areceptor); and arresting, which also deactivate and desensitizereceptors. Modulators can include genetically modified versions of T1Rfamily members, e.g., with altered activity, as well as naturallyoccurring and synthetic ligands, antagonists, agonists, small chemicalmolecules and the like. Such assays for inhibitors and activatorsinclude, e.g., expressing T1R family members in cells or cell membranes,applying putative modulator compounds, in the presence or absence oftastants, e.g., sweet tastants, and then determining the functionaleffects on taste transduction, as described above. Samples or assayscomprising T1R family members that are treated with a potentialenhancer, activator, inhibitor, or modulator are compared to controlsamples without the inhibitor, activator, or modulator to examine theextent of modulation. Positive control samples (e.g. a sweet tastantwithout added modulators) are assigned a relative T1R activity value of100%.

“EC₅₀” is defined as the amount of a compound that elicits 50% of themaximal response the compound can elicit, whether as an activator,enhancer, or modulator. A dose-dependent response curve was determinedfor a compound, and the compound concentration corresponding to 50% ofthe maximal response was derived from the curve, in one example.

“IC₅₀” is defined as the amount of a compound that elicits 50% of themaximal effect the compound can elicit as an inhibitor.

Regarding sweet tastants and enhancers, after a compound is identified,scores of their activities are given as percentage of the maximumfructose intensity (%). In compound dose response, an EC₅₀ can becalculated to reflect the potency of the compound as a sweet agonist. Inthe present invention, an EC₅₀ of lower than about 100 mM is indicativeof compounds that induce T1R2/T1R3 activity as a sweet agonist.Preferably, a positive hit for a sweet agonist has an EC₅₀ value of lessthan about 1 mM; more preferably less than about a 10 μM.

In sweet enhancement assay experiments, a fructose dose response was runand a second fructose dose response was run with a certain amount ofcandidate compound at every fructose concentrations at the same time.Then, the EC₅₀ ratio can be calculated based on the followingdefinitions:EC ₅₀Ratio=EC ₅₀(fructose)/EC ₅₀(fructose+[Compound])

wherein “[compound]” refers to the concentration of compound used toelicit (or enhance or potentiate) the fructose dose response. Thoseconcentrations could vary from a pM to an mM, more preferred, from a lownM to μM. A potent sweet enhancer would have a high EC₅₀ Ratio at a lowconcentration of the compound used.

In the present invention, an EC₅₀ ratio of greater than 1 is indicativeof a compound that modulates (potentiates) T1R2/T1R3 activity and is answeet enhancer. Preferably, a positive hit will have EC₅₀ ratio valuesof at least 1.20, preferably ranging from at least 1.50 to 100 or evenhigher.

By contrast, competing agonists (those sweet tastants that bind mutuallyexclusively) or inhibitors always yield values of EC₅₀ ratio less than1, such as from 0-1.

Regarding umami tastants and enhancers, scores of their activities canbe given as percentage of the maximum MSG intensity (%). In compounddose response, an EC₅₀ can be calculated to reflect the potency of thecompound as umami agonist. In the present invention, an EC₅₀ of lowerthan about 10 mM is indicative of compounds that induce T1R1/T1R3activity and an umami agonist. Preferably, a positive hit for an umamiagonist will have EC₅₀ values of less than about 1 mM; more preferablyranging from about a pM to about a low μM.

In enhancement assay experiments, a MSG dose response was run and asecond. MSG dose response was run with a certain amount of candidatecompound at every MSG concentrations at the same time. Then, the EC₅₀ratio is calculated based on the following definitions:EC ₅₀Ratio=EC ₅₀(MSG)/EC ₅₀(MSG+[Compound])

wherein “[compound]” refers to the concentration of compound used toelicit (or enhance or potentiate) the MSG, dose response. Thoseconcentrations can vary from a pM to an mM, more preferred, from a lownM to μM. A potent umami enhancer has a high EC₅₀ Ratio at a lowconcentration of the compound used.

In the present invention, an EC₅₀ ratio of greater than 1 is indicativeof a compound that modulates (potentiates) T1R1/T1R3 activity and in anumami enhancer. Preferably, a positive hit has EC₅₀ ratio values of atleast 1.20, preferably ranging from at least 1.50 to 100 or even higher.

Negative control samples (e.g. buffer without an added taste stimulus)are assigned a relative T1R activity value of 0%. Inhibition of a T1R isachieved when a mixture of the positive control sample and a modulatorresult in the T1R activity value relative to the positive control isabout 80%, optionally 50% or 25-0%. Activation of a T1R by a modulatoralone is achieved when the T1R activity value relative to the positivecontrol sample is 10%, 25%, 50%, 75%, optionally 100%, optionally 150%,optionally 200-500%, or 1000-3000% higher.

The terms “purified,” “substantially purified,” and “isolated” as usedherein refer to the state of being free of other, dissimilar compoundswith which the compound of the invention is normally associated in itsnatural state, so that the “purified,” “substantially purified,” and“isolated” subject comprises at least 0.5%, 1%, 5%, 10%, or 20%, andmost preferably at least 50% or 75% of the mass, by weight, of a givensample. In one preferred embodiment, these terms refer to the compoundof the invention comprising at least 95%, of the mass, by weight, of agiven sample. As used herein, the terms “purified,” “substantiallypurified,” and “isolated,” when referring to a nucleic acid or protein,also refers to a state of purification or concentration different thanthat which occurs naturally in the mammalian, especially human body. Anydegree of purification or concentration greater than that which occursnaturally in the mammalian, especially human, body, including (1) thepurification from other associated structures or compounds or (2) theassociation with structures or compounds to which it is not normallyassociated in the mammalian, especially human, body, are within themeaning of “isolated.” The nucleic acid or protein or classes of nucleicacids or proteins, described herein, may be isolated, or otherwiseassociated with structures or compounds to which they are not, normallyassociated in nature, according to a variety of methods and processesknown to those of skill in the art.

The term “nucleic acid” or “nucleic acid sequence” refers to adeoxy-ribonucleotide or ribonucleotide oligonucleotide in either single-or double-stranded form. The term encompasses nucleic acids, i.e.,oligonucleotides, containing known analogs of natural nucleotides. Theterm also encompasses nucleic-acid-like structures with syntheticbackbones (see e.g., Oligonucleotides and Analogues, a PracticalApproach, ed. F. Eckstein, Oxford Univ. Press (1991); AntisenseStrategies, Annals of the N.Y. Academy of Sciences, Vol. 600, Eds.Baserga et al. (NYAS 1992); Milligan J. Med. Chem. 36:1923-1937 (1993);Antisense Research and Applications (1993, CRC Press), WO 97/03211; WO96/39154; Mata, Toxicol. Appl. Pharmacol. 144:189-197 (1997);Strauss-Soukup, Biochemistry 36:8692-8698 (1997); Samstag, AntisenseNucleic Acid Drug Dev, 6:153-156 (1996)).

Unless otherwise indicated, a particular nucleic acid sequence alsoimplicitly encompasses conservatively modified variants thereof (e.g.,degenerate codon substitutions) and complementary sequences, as well asthe sequence explicitly indicated. Specifically, degenerate codonsubstitutions may be achieved by generating, e.g., sequences in whichthe third position of one or more selected codons is substituted withmixed-base and/or deoxyinosine residues (Batzer et al., Nucleic AcidRes., 19:5081 (1991); Ohtsuka et al., J. Biol. Chem., 260:2605-2608(1985); Rossolini et al., Mol. Cell. Probes, 8:91-98 (1994)). The termnucleic acid is used interchangeably with gene, cDNA, mRNA,oligonucleotide, and polynucleotide.

The terms “polypeptide,” “peptide” and “protein” are usedinterchangeably herein to refer to a polymer of amino acid residues. Theterms apply to amino acid polymers in which one or more amino acidresidue is an artificial chemical mimetic of a corresponding naturallyoccurring amino acid, as well as to naturally occurring amino acidpolymers and non-naturally occurring amino acid polymer.

The term “plasma membrane translocation domain” or simply “translocationdomain” means a polypeptide domain that, when incorporated into apolypeptide coding sequence, can with greater efficiency “chaperone” or“translocate” the hybrid (“fusion”) protein to the cell plasma membranethan without the domain. For instance, a “translocation domain” may bederived from the amino terminus of the bovine rhodopsin receptorpolypeptide, a 7-transmembrane receptor. However, rhodopsin from anymammal may be used, as can other translocation facilitating sequences.Thus, the translocation domain is particularly efficient intranslocating 7-transmembrane fusion proteins to the plasma membrane,and a protein (e.g., a taste receptor polypeptide) comprising an aminoterminal translocating domain will be transported to the plasma membranemore efficiently than without the domain. However, if the N-terminaldomain of the polypeptide is active in binding, as with the T1Rreceptors of the present invention, the use of other translocationdomains may be preferred. For instance, a PDZ domain-interactingpeptide, as described herein, may be used.

The “translocation domain,” “ligand-binding domain”, and chimericreceptors compositions described herein also include “analogs,” or“conservative variants” and “mimetics” (“peptidomimetics”) withstructures and activity that substantially correspond to the exemplarysequences. Thus, the terms “conservative variant” or “analog” or“mimetic” refer to a polypeptide which has a modified amino acidsequence, such that the change(s) do not substantially alter thepolypeptide's (the conservative variant's) structure and/or activity, asdefined herein. These include conservatively modified variations of anamino acid sequence, i.e., amino acid substitutions, additions ordeletions of those residues that are not critical for protein activity,or substitution of amino acids with residues having similar properties(e.g., acidic, basic, positively or negatively charged, polar ornon-polar, etc.) such that the substitutions of even critical aminoacids does not substantially alter structure and/or activity.

More particularly, “conservatively modified variants” applies to bothamino acid and nucleic acid sequences. With respect to particularnucleic acid sequences, conservatively modified variants refers to thosenucleic acids which encode identical or essentially identical amino acidsequences, or where the nucleic acid does not encode an amino acidsequence, to essentially identical sequences. Because of the degeneracyof the genetic code, a large number of functionally identical nucleicacids encode any given protein.

For instance, the codons GCA, GCC, GCG and GCU all encode the amino acidalanine. Thus, at every position where an alanine is specified by acodon, the codon can be altered to any of the corresponding codonsdescribed without altering the encoded polypeptide.

Such nucleic acid variations are “silent variations,” which are onespecies of conservatively modified variations. Every nucleic acidsequence herein, which encodes a polypeptide, also describes everypossible silent variation of the nucleic acid. One of skill willrecognize that each codon in a nucleic acid (except AUG, which isordinarily the only codon for methionine, and TGG, which is ordinarilythe only codon for tryptophan) can be modified to yield a functionallyidentical molecule. Accordingly, each silent variation of a nucleicacid, which encodes a polypeptide, is implicit in each describedsequence.

Conservative substitution tables providing functionally similar aminoacids are well known in the art. For example, one exemplary guideline toselect conservative substitutions includes (original residue followed byexemplary substitution): ala/gly or ser; arg/lys; asn/gln or his;asp/glu; cys/ser; gln/asn; gly/asp; gly/ala or pro; his/asn or gln;ile/leu or val; leu/ile or val; lys/arg or gln or glu; met/leu or tyr orile; phe/met or leu or tyr; ser/thr; thr/ser; trp/tyr; tyr/trp or phe;val/ile or leu. An alternative exemplary guideline uses the followingsix groups, each containing amino acids that are conservativesubstitutions for one another: 1) Alanine (A), Serine (S), Threonine(T); 2) Aspartic acid (D), Glutamic acid (E); 3) Asparagine (N),Glutamine (Q); 4) Arginine (R), Lysine (I); 5) Isoleucine (I), Leucine(L), Methionine (M), Valine (V); and 6) Phenylalanine (F), Tyrosine (Y),Tryptophan (W); (see also, e.g., Creighton, Proteins, W.H. Freeman andCompany (1984); Schultz and Schimer, Principles of Protein Structure,Springer-Verlag (1979)). One of skill in the art will appreciate thatthe above-identified substitutions are not the only possibleconservative substitutions. For example, for some purposes, one mayregard all charged amino acids as conservative substitutions for eachother whether they are positive or negative. In addition, individualsubstitutions, deletions or additions that alter, add or delete a singleamino acid or a small percentage of amino acids in an encoded sequencecan also be considered “conservatively modified variations.”

The terms “mimetic” and “peptidomimetic” refer to a synthetic chemicalcompound that has substantially the same structural and/or functionalcharacteristics of the polypeptides, e.g., translocation domains,ligand-binding domains, or chimeric receptors of the invention. Themimetic can be either entirely composed of synthetic, non-naturalanalogs of amino acids, or may be a chimeric molecule of partly naturalpeptide amino acids and partly non-natural analogs of amino acids. Themimetic can also incorporate any amount of natural amino acidconservative substitutions as long as such substitutions also do notsubstantially alter the mimetic's structure and/or activity.

As with polypeptides of the invention which are conservative variants,routine experimentation will determine whether a mimetic is within thescope of the invention, i.e., that its structure and/or function is notsubstantially altered. Polypeptide mimetic compositions can contain anycombination of non-natural structural components, which are typicallyfrom three structural groups: a) residue linkage groups other than thenatural amide bond (“peptide bond”) linkages; b) non-natural residues inplace of naturally occurring amino acid residues; or c) residues whichinduce secondary structural mimicry, i.e., to induce or stabilize asecondary structure, e.g., a beta turn, gamma turn, beta sheet, alphahelix, conformation, and the like. A polypeptide can be characterized asa mimetic when all or some of its residues are joined by chemical meansother than natural peptide bonds. Individual peptidomimetic residues canbe joined by peptide bonds, other chemical bonds or coupling means, suchas, e.g., glutaraldehyde, N-hydroxysuccinimide esters, bifunctionalmaleimides, N,N′-dicyclohexylcarbodiimide (DCC) orN,N′-diisopropylcarbodiimide (DIC). Linking groups that can be analternative to the traditional amide bond (“peptide bond”) linkagesinclude, e.g., ketomethylene (e.g., —C(═O)—CH₂— for —C(═O)—NH—),aminomethylene (CH₂—NH), ethylene, olefin (CH═CH), ether (CH₂—O),thioether (CH₂—S), tetrazole (CN₄), thiazole, retroamide, or ester (see,e.g., Spatola, Chemistry and Biochemistry of Amino Acids, Peptides andProteins, Vol. 7, pp 267-357, “Peptide Backbone Modifications,” MarcellDekker, NY (1983)). A polypeptide can also be characterized as a mimeticby containing all or some non-natural residues in place of naturallyoccurring amino acid residues; non-natural residues are well describedin the scientific and patent literature.

A “label” or a “detectable moiety” is a composition detectable byspectroscopic, photochemical, biochemical, immunochemical, or chemicalmeans. For example, useful labels include ³²P, fluorescent dyes,electron-dense reagents, enzymes (e.g., as commonly used in an ELISA),biotin, digoxigenin, or haptens and proteins which can be madedetectable, e.g., by incorporating a radiolabel into the peptide or usedto detect antibodies specifically reactive with the peptide.

A “labeled nucleic acid probe or oligonucleotide” is one that is bound,either covalently, through a linker or a chemical bond, ornoncovalently, through ionic, van der Waals, electrostatic, or hydrogenbonds to a label such that the presence of the probe may be detected bydetecting the presence of the label bound to the probe.

As used herein a “nucleic acid probe or oligonucleotide” is defined as anucleic acid capable of binding to a target nucleic acid ofcomplementary sequence through one or more types of chemical bonds,usually through complementary base pairing, usually through hydrogenbond formation. As used herein, a probe may include natural (i.e., A, G,C, or T) or modified bases (7-deazaguanosine, inosine, etc.). Inaddition, the bases in a probe may be joined by a linkage other than aphosphodiester bond, so long as it does not interfere withhybridization. Thus, for example, probes may be peptide nucleic acids inwhich the constituent bases are joined by peptide bonds rather thanphosphodiester linkages. It will be understood by one of skill in theart that probes may bind target sequences lacking completecomplementarity with the probe sequence depending upon the stringency ofthe hybridization conditions. The probes are optionally directly labeledas with isotopes, chromophores, lumiphores, chromogens, or indirectlylabeled such as with biotin to which a streptavidin complex may laterbind. By assaying for the presence or absence of the probe, one candetect the presence or absence of the select sequence or subsequence.

The term “heterologous” when used with reference to portions of anucleic acid indicates that the nucleic acid comprises two or moresubsequences that are not found in the same relationship to each otherin nature. For instance, the nucleic acid is typically recombinantlyproduced, having two or more sequences from unrelated genes arranged tomake a new functional nucleic acid, e.g., a promoter from one source anda coding region from another source. Similarly, a heterologous proteinindicates that the protein comprises two or more subsequences that arenot found in the same relationship to each other in nature (e.g., afusion protein).

A “promoter” is defined as an array of nucleic acid sequences thatdirect transcription of a nucleic acid. As used herein, a promoterincludes necessary nucleic acid sequences near the start site oftranscription, such as, in the case of a polymerase II type promoter, aTATA element. A promoter also optionally includes distal enhancer orrepressor elements, which can be located as much as several thousandbase pairs from the start site of transcription. A “constitutive”promoter is a promoter that is active under most environmental anddevelopmental conditions.

An “inducible” promoter is a promoter that is active under environmentalor developmental regulation. The term “operably linked” refers to afunctional linkage between a nucleic acid expression control sequence(such as a promoter, or array of transcription factor binding sites) anda second nucleic acid sequence, wherein the expression control sequencedirects transcription of the nucleic acid corresponding to the secondsequence.

As used herein, “recombinant” refers to a polynucleotide synthesized orotherwise manipulated in vitro (e.g., “recombinant polynucleotide”), tomethods of using recombinant polynucleotides to produce gene products incells or other biological systems, or to a polypeptide (“recombinantprotein”) encoded by a recombinant polynucleotide. “Recombinant means”also encompass the ligation of nucleic acids having various codingregions or domains or promoter sequences from different sources into anexpression cassette or vector for expression of, e.g., inducible orconstitutive expression of a fusion protein comprising a translocationdomain of the invention and a nucleic acid sequence amplified using aprimer of the invention.

As used herein, a “stable cell line” refers to a cell line, whichstably, i.e. over a prolonged period, expresses a heterologous nucleicsequence, i.e. a T1R or G protein. In preferred embodiments, such stablecell lines will be produced by transfecting appropriate cells, typicallymammalian cells, e.g. HEK-293 cells, with a linearized vector thatcontains a T1R expression construct, i.e. T1R1, T1R2 and/or T1R3. Mostpreferably, such stable cell lines will be produced by co-transfectingtwo linearized plasmids that express hT1R1 and hT1R3 or hT1R2 and hT1R3and an appropriate selection procedure to generate cell lines havingthese genes stably integrated therein. Most preferably, the cell linewill also stably express a G protein such as G_(α15).

The phrase “selectively (or specifically) hybridizes to” refers to thebinding, duplexing, or hybridizing of a molecule only to a particularnucleotide sequence under stringent hybridization conditions when thatsequence is present in a complex mixture (e.g., total cellular orlibrary DNA or RNA).

The phrase “stringent hybridization conditions” refers to conditionsunder which a probe will hybridize to its target subsequence, typicallyin a complex mixture of nucleic acid, but to no other sequences.Stringent conditions are sequence dependent and will be different indifferent circumstances. Longer sequences hybridize specifically athigher temperatures. An extensive guide to the hybridization of nucleicacids is found in Tijssen, Techniques in Biochemistry and MolecularBiology—Hybridization with Nucleic Probes, “Overview of principles ofhybridization and the strategy of nucleic acid assays” (1993).Generally, stringent conditions are selected to be about 5-10° C. lowerthan the thermal melting point (Tm) for the specific sequence at adefined ionic strength pH. The Tm is the temperature (under definedionic strength, pH, and nucleic concentration) at which 50% of theprobes complementary to the target hybridize to the target sequence atequilibrium (as the target sequences are present in excess, at Tm, 50%of the probes are occupied at equilibrium). Stringent conditions will bethose in which the salt concentration is less than about 1.0 M sodiumion, typically about 0.01 to 1.0 M sodium ion concentration (or othersalts) at pH 7.0 to 8.3 and the temperature is at least about 30° C. forshort probes (e.g., 10 to 50 nucleotides) and at least about 60° C. forlong probes (e.g., greater than 50 nucleotides). Stringent conditionsmay also be achieved with the addition of destabilizing agents such asformamide. For selective or specific hybridization, a positive signal isat least two times background, optionally 10 times backgroundhybridization. Exemplary stringent hybridization conditions can be asfollowing: 50% formamide, 5×SSC, and 1% SDS, incubating at 42° C., or,5×SSC, 1% SDS, incubating at 65° C., with wash in 0.2×SSC, and 0.1% SDSat 65° C. Such hybridizations and wash steps can be carried out for,e.g., 1, 2, 5, 10, 15, 30, 60; or more minutes.

Nucleic acids that do not hybridize to each other under stringentconditions are still substantially related if the polypeptides that theyencode are substantially related. This occurs, for example, when a copyof a nucleic acid is created using the maximum codon degeneracypermitted by the genetic code. In such cases, the nucleic acidstypically hybridize under moderately stringent hybridization conditions.Exemplary “moderately stringent hybridization conditions” include ahybridization in a buffer of 40% formamide, 1 M NaCI, 1% SDS at 37° C.,and a wash in 1×SSC at 45° C. Such hybridizations and wash steps can becarried out for, e.g., 1, 2, 5, 10, 15, 30, 60, or more minutes. Apositive hybridization is at least twice background. Those of ordinaryskill will readily recognize that alternative hybridization and washconditions can be utilized to provide conditions of similar stringency.

“Antibody” refers to a polypeptide comprising a framework region from animmunoglobulin gene or fragments thereof that specifically binds andrecognizes an antigen. The recognized immunoglobulin genes include thekappa, lambda, alpha, gamma, delta, epsilon, and mu constant regiongenes, as well as the myriad immunoglobulin variable region genes. Lightchains are classified as either kappa or lambda. Heavy chains areclassified as gamma, mu, alpha, delta, or epsilon, which in turn definethe immunoglobulin classes, IgG, IgM, IgA, IgD and IgE, respectively.

An exemplary immunoglobulin (antibody) structural unit comprises atetramer. Each tetramer is composed of two identical pairs ofpolypeptide chains, each pair having one “light” (about 25 kDa) and one“heavy” chain (about 50-70 kDa). The N-terminus of each chain defines avariable region of about 100 to 110 or more amino acids primarilyresponsible for antigen recognition. The terms “variable light chain”(VL) and “variable heavy chain” (VH) refer to these light and heavychains respectively.

A “chimeric antibody” is an antibody molecule in which (a) the constantregion, or a portion thereof, is altered, replaced or exchanged so thatthe antigen binding site (variable region) is linked to a constantregion of a different or altered class, effector function and/orspecies, or an entirely different molecule which confers new propertiesto the chimeric antibody, e.g., an enzyme, toxin, hormone, growthfactor, drug, etc.; or (b) the variable region, or a portion thereof, isaltered, replaced or exchanged with a variable region having a differentor altered antigen specificity.

An “anti-T1R” antibody is an antibody or antibody fragment thatspecifically binds a polypeptide encoded by a T1R gene, cDNA, or asubsequence thereof.

The term “immunoassay” is an assay that uses an antibody to specificallybind an antigen. The immunoassay is characterized by the use of specificbinding properties of a particular antibody to isolate, target, and/orquantify the antigen.

The phrase “specifically (or selectively) binds” or “specifically (orselectively) reacts with,” when referring to a molecule or composition,refers to a binding reaction that is determinative of the presence ofthe molecule in a heterogeneous population of other biologics. Thus,under designated conditions, the specified molecules bind to aparticular receptor at least two times the background and do notsubstantially bind in a significant amount to other molecules present inthe sample. Specific binding to a receptor under such conditions mayrequire a receptor that is selected for its specificity for a particularmolecule.

Regarding antibodies, a variety of immunoassay formats may be used toselect antibodies specifically immunoreactive with a particular protein.For example, solid-phase ELISA immunoassays are routinely used to selectantibodies specifically immunoreactive with a protein (see, e.g., Harlow& Lane, Antibodies, A Laboratory Manual, (1988), for a description ofimmunoassay formats and conditions that can be used to determinespecific immunoreactivity). Typically a specific or selective reactionwill be at least twice background signal or noise and more typicallymore than 10 to 100 times background.

The phrase “selectively associates with” refers to the ability of anucleic acid to “selectively hybridize” with another as defined above,or the ability of an antibody to “selectively (or specifically) bind toa protein, as defined above.

The term “expression vector” refers to any recombinant expression systemfor the purpose of expressing a nucleic acid sequence of the inventionin vitro or in vivo, constitutively or inducibly, in any cell, includingprokaryotic, yeast, fungal, plant, insect or mammalian cell. The termincludes linear or circular expression systems. The term includesexpression systems that remain episomal or integrate into the host cellgenome. The expression systems can have the ability to self-replicate ornot, i.e., drive only transient expression in a cell. The term includesrecombinant expression “cassettes which contain only the minimumelements needed for transcription of the recombinant nucleic acid.

By “host cell” is meant a cell that contains an expression vector andsupports the replication or expression of the expression vector. Hostcells may be prokaryotic cells such as E. coli, or eukaryotic cells suchas yeast, insect, amphibian, worm or mammalian cells such as CHO, Hela,HEK-293, and the like, e.g., cultured cells, explants, and cells invivo.

Compounds

As discussed above, there are different domains on the T1R receptors.T1R1, T1R2, and T1R3 each contain an N-terminal extracellular domain(also known as the Venus flytrap domain), transmembrane domainscomprising seven transmembrane regions, and corresponding cytoplasmic,and extracellular loops; a cysteine rich domain, and a C-terminaldomain. Each region defines a specific set of compounds that bindspecifically to that region.

In humans, the N-terminal extracellular domain comprises amino acids 1to 560 of hT1R2 and amino acids 1 to 563 of hT1R3. In rats, theN-terminal extracellular domain comprises amino acids 1 to 564 of rT1R2,and amino acids 1 to 568 of rT1R3.

In humans, the C-terminal transmembrane domain and intracellular domaincomprise amino acids 561 to 839 of hT1R2, and amino acids 564 to 852 ofhT1R3. In rats, the C-terminal transmembrane domain and intracellulardomain comprise amino acids 565 to 842 of rT1R2, and amino acids 569 to858 of rT1R3.

Metabotropic glutamate receptors (mGluR) are another class of C-class Gprotein-coupled receptors that respond to glutamate. These are foundprimarily in the brain and neuronal tissue where they play a role inneuronal signaling. The mGluR N-terminal extracellular domain can becovalently linked to a T1R in order to create chimeric receptors. ThemGluR receptor can be any of mGluR1-mGluR8, for example. Differentligands bind to different domains on different subunits of both theumami and the sweet receptors. For example, aspartame and neotame bindto the N-terminal extracellular domain of T1R2, while cyclamate,neohesperidin dihydrochalcone (NHDC), and lactisole bind to thetransmembrane domain of T1R3. Because T1R3 is one of the two subunits inthe T1R1/T1R3 umami taste receptor, cyclamate, NHDC and lactisole caninteract with T1R3 in the T1R1/T1R3 umami taste receptor as well.Cyclamate and NHDC enhance the activity of the umami taste receptor,while lactisole inhibits the umami receptor.

The specific binding compounds of the invention as it relates to umamitastants comprise amides. The amide compounds also comprise certainsub-classes of amide derivatives or classes of derivatives related toamides, such as for example ureas, urethanes, oxalamides, acrylamides,and the like.

Molecules that interact with the transmembrane domain of T1R2, forexample, can be modulators of sweet taste, and molecules that interactwith the transmembrane domain of T1R3 can be modulators of sweet tasteand/or umami taste.

Human T1R2/T1R3 recognizes a group of sweeteners which are notrecognized by rat T1R2/T1R3, and human but not rat T1R2/T1R3 isinhibited by lactisole. When the extracellular domain of human T1R2 wasreplaced by its rat counterpart, the human receptor lost the ability torecognize aspartame, indicating that this part of human T1R2 is requiredfor binding to aspartame. Inversely, when the extracellular domain ofrat T1R2 was replaced by its human counterpart, the rat receptoracquired the ability to recognize aspartame, indicating that this partof the human T1R2 is sufficient to bind aspartame. By the sameprinciple, the transmembrane domain of human T1R3 was required andsufficient for

Table 6 shows the abbreviations used to represent various rat/humanchimeric receptors and receptor subunits.

TABLE 6 hT1R2 - human T1R2 hT1R3 - human T1R3 rT1R2 - rat T1R2 rT1R3 -rat T1R3 hT1R2/rT1R3 - a receptor composed of human T1R2 and rat T1R3rT1R2/hT1R3 - a receptor composed of a rat T1R2 and human T1R3hT1R2/h3-r3 - a receptor composed of human T1R2 and a chimeric T1R3 withhuman N- terminal extracellular domain and rat transmembrane andC-terminal domain rT1R2/r3-h3 - a receptor composed of rat T1R2 and achimeric T1R3 with rat N-terminal extracellular domain and humantransmembrane and C-terminal domain h2-r2/rT1R3 - a receptor composed ofa chimeric T1R2 with human N-terminal extracellular domain and rattransmembrane and C-terminal domain and rat T1R3 r2-h2/rT1R3 - areceptor composed of a chimeric T1R2 with rat N-terminal extracellulardomain and human transmembrane and C-terminal domain and rat T1R3h2-h1/hT1R3 - a receptor composed of a chimeric T1R with human T1R2N-terminal extracellular domain and human T1R1 transmembrane andC-terminal domain and human T1R3 h1-h2/hT1R3 - a receptor composed of achimeric T1R with human T1R1 N-terminal extracellular domain and humanT1R2 transmembrane and C-terminal domain and human T1R3h2-mGluR/h3-mGluR1 - a receptor composed of a N-terminal extracellulardomain from hT1R2 covalently linked to the transmembrane and C-terminaldomain of mGluR1 and a N-terminal extracellular domain from hT1R3covalently linked to the transmembrane and C-terminal domain of mGluR1h1-mGlu1R/h3-mGluR1 - a receptor composed of a N-terminal extracellulardomain from hT1R1 covalently linked to the transmembrane and C-terminaldomain of mGluR1 and a N-terminal extracellular domain from hT1R3covalently linked to the transmembrane and C-terminal domain of mGluR1mGluR1-h2/mGluR1-h3 - a receptor composed of a N-terminal extracellulardomain from mGluR1 covalently linked to the transmembrane and C-terminaldomain of hT1R2 and a N-terminal extracellular domain from a mGluR1covalently linked to the transmembrane and C-terminal domain of hT1R3mGluR1-h1/mGluR1-h3 - receptor composed of a N-terminal extracellulardomain from mGluR1 covalently linked to the transmembrane and C-terminaldomain of hT1R1 and a N-terminal extracellular domain from mGluR1covalently linked to the transmembrane and C-terminal domain of hT1R3

Disclosed herein are non-naturally occurring compounds that specificallybind to the T1R2/T1R3 receptor comprising hT1R2/hT1R3 but notrT1R2/rT1R3. Examples of such compounds include, but are not limited toneotame, aspartame, cyclamate, lactisol, Compound 883360, Compound6542888, Compound 403249, Compound 6364395, Dihydroxybenzoic acid (DHB),Compound 6542888, and neohesperidin dihydrochalcone (NHDC) Additionalexamples are found in Tables 1-4. The organic, non-peptide compounds canbe approximately the size of a box of dimensions 15×8×8 angstroms, morepreferably the dimension should be 12×5×5 angstroms.

Also disclosed are compounds that specifically bind to a T1R2/T1R3receptor comprising hT1R2/rT1R3 but not rT1R2/hT1R3. Examples of suchcompounds include, but are not limited to aspartame, and neotame.Additional examples are found in Table 5.

Also disclosed are compounds that specifically bind to the N-terminalextracellular domain of T1R2 of the hT1R2/hT1R3 receptor. Examples ofsuch compounds include, but are not limited to neotame, aspartamecarbohydrate sugars (e.g. sucrose, fructose, glucose, tagatose,erythritol, sorbitol, maltose, xylitol, lactose and galactose, as wellas all other carbohydrate sugars). Additional examples are found inTable 5.

Also disclosed are compounds that specifically bind to the Venus FlytrapDomain (VFD) of T1R2 of the hT1R2/hT1R3 and hT1R2/rT1R3 receptor.

Also disclosed are compounds that specifically bind to the N-terminalVenus flytrap domain of the T1R2 subunit of the T1R2/T1R3 receptor. Morespecifically, also disclosed are compounds that specifically bind toamino acid residues 144 and 302 of the human N-terminal Venus flytrapdomain of the T1R2 subunit of the T1R2/T1R3 receptor. Examples of suchcompounds include, but are not limited to aspartame, neotame,carbohydrates, and sweet amino acids, such as D-Trp, Ala, and Gly.

Also disclosed are compounds that specifically bind to the cysteine-richregion of T1R2 of the hT1R2/hT1R3 receptor. Also disclosed are compoundsthat specifically bind to the Transmembrane Domain (TM) of T1R2 of thehT1R2/hT1R3 receptor.

Also disclosed are compounds that specifically bind to a T1R2/T1R3receptor comprising rT1R2/hT1R3 but not hT1R2/rT1R3. Examples of suchcompounds include, but are not limited to cyclamate, NHDC, lactisole,Compound 883360, Compound 403249, and Compound 6364395. Additionalexamples are found in Table 5.

Also disclosed are compounds that specifically bind to hT1R2/hT1R3 andrT1R2/r3-h3 but not to rT1R2/rT1R3 or to hT1R2/h3-r3. Examples of suchcompounds include, but are not limited to cyclamate, NHDC, lactisole,Compound 883360, Compound 403249 and Compound 6364395.

Also disclosed are compounds that specifically bind to extracellularloop 2 and extracellular loop 3 of the human C-terminal domain of theT1R3 subunit of the T1R2/T1R3 receptor. Also disclosed are compoundsthat specifically bind to hT1R2/hT1R3 and r2-h2/rT1R3 but not torT1R2/rT1R3 or to h2-r2/hT1R3.

Also disclosed are compounds that specifically bind to the humanN-terminal extracellular domain of the T1R3 subunit of the T1R2/T1R3receptor. Also disclosed are compounds that specifically bind to theVenus Flytrap Domain (VFD) of T1R3 of the hT1R2/hT1R3 receptor. Examplesof such compounds include, but are not limited to aspartame, neotame,carbohydrates, and sweet amino acids, such as D-Trp, Ala, and Gly.

Also disclosed are compounds that specifically bind to the TransmembraneDomain of T1R3 of the hT1R2/hT1R3 receptor. Also disclosed are compoundsthat specifically bind to extracellular loop 2 and extracellular loop 3of the human transmembrane domain of the T1R3 subunit of T1R2/T1R3.Examples of such compounds include, but are not limited to cyclamate.

The compound of the invention does not include sucrose, fructose,glucose, erythritol, isomalt, lactitol, mannitol, sorbitol, xylitol,certain known natural terpenoids, flavonoids, or protein sweeteners,di-peptides, tri-peptides, aspartame, saccharin, sucralose, halogenatedsaccharides, acesulfame-K, cyclamate, sucralose, and alitame. neotame,perillartine, SC-45647, SC-40014, monellin, NC-002740-01, thaumatin,CC-00100, NC-00420, alitame, SC-44102, dulcin, NC-00576, slycyrrhizicAcid, stevioside, Na-Saccharin, D-tryptophan, cyclamate, DHB, glycolicAcid, glycine, D(−)fructose, homofuronol, D(−)tagatose, maltose, D(+)glucose, D-sorbitol, D(+)galactose, α-lactose, L( )fructose,L(+)Compound 403249, and glucose.

Optionally, a compound of the invention is also not Compound 6364395.

Also disclosed herein are compounds that bind a truncated region of aT1R domain. For example, disclosed are compounds that specifically bindto the TM domain of T1R2 of a truncated sweet receptor comprisingh2TM/h3TM, compounds that specifically bind to the TM domain of T1R3 ofa truncated sweet receptor comprising h2TM/h3TM, compounds thatspecifically bind to the TM domain of T1R2 of a chimeric receptorcomprising mGluR-h2/mGluR-h3, compounds that specifically bind to the TMdomain of T1R3 of a chimeric receptor comprising mGluR-h2/mGluR-h3,compounds that binds to the TM domain of T1R1 of a truncated savoryreceptor comprising h1TM/h3™, compound that binds to the TM domain ofT1R3 of a truncated sweet receptor comprising h1TM/h3™, compounds thatbind to the TM domain of T1R1 of a chimeric receptor comprisingmGluR-h1/mGluR-h3, and compounds that bind to the TM domain of T1R3 of achimeric receptor comprising mGluR-h1/mGluR-h3. SEQ ID NOS: 29-33represent these truncated receptors.

The compounds of the invention do not include monosodium glutamate(“MSG”), inosine monophosphate (IMP) or guanosine monophosphate (GMP),sucrose, fructose, glucose, erythritol, isomalt, lactitol, mannitol,sorbitol, xylitol, certain known natural terpenoids, flavonoids, orprotein sweeteners, di-peptides, tri-peptides aspartame, saccharin,sucralose, halogenated saccharides, acesulfame-K, cyclamate, sucralose,alitame, monosodium glutamate (“MSG”), inosine monophosphate (IMP) orguanosine monophosphate (GMP), or adenosine monophosphate.

Compound 403249 is (5-(4H-benzo[d][1,3]oxathiin-2-yl)-2-methyoxyphenol,while Compound 6364395 is3-(3-hydroxy-4-methoxyphenethyl)benzo[d]isoxyazole-4,6-diol.

The compounds described above can demonstrate a compound-dependentincrease in fluorescence with an activity compared to the maximalactivity for fructose of at least 25% in a fluorescence-based assayusing a FLIPR instrument (Fluorometric Intensity Plate Reader, MolecularDevices, Sunnyvale, Calif.) For examples of this protocol, see Examples12 and 18. The compounds can also demonstrate a compound-dependentdecrease in the EC50 for a sweetener by at least two-fold in afluorescence-based assay using a FLIPR (Molecular Devices) instrument.Furthermore, in a cell-based assay, the compound can result in at least10 out of 100 cells transfected with wild-type or chimeric receptorshowing a compound-dependent increase in fluorescence. An example of acell-based assay can be found in Example 24. The compound can alsodemonstrate a compound-dependent increase of at least 1.1, 1.2, 1.3,1.4, 1.5, 1.6, 1.7, 1.8, 1.9, two-fold or greater, or any number inbetween, in the number of fluorescent cells in response to a sub-maximallevel of a sweetener. The response can be measured by fluorescence,calcium levels, IP3 levels, cAMP levels, GTPγS binding, or reporter geneactivity (e.g. luciferase, beta-galactosidase).

Furthermore, the compounds disclosed herein can have one or more of thefollowing characteristics in a cell: a decreased EC₅₀ compared to acontrol of at least approximately 50%, increased intracellular Ca²⁺level by at least approximately 25%, increased intracellular cAMP by atleast approximately 25%, increased intracellular cGMP by at leastapproximately 25%, increased intracellular IP₃ by at least approximately25%, or increased G protein binding of GTPγS by at least approximately25%.

Methods of Using the Compounds

Also disclosed are methods modulating the savory taste of a comestibleor medicinal product comprising providing at least one comestible ormedicinal product, or a precursor thereof, and combining the comestibleor medicinal product or precursor thereof with at least a savory flavormodulating amount of at least one non-naturally occurring compound asdisclosed herein, or a comestibly acceptable salt thereof, so as to forma modified comestible or medicinal product; thereby modulating thesavory taste of a comestible or medicinal product.

Also disclosed are methods for inhibiting the savory taste of acomestible or medicinal product comprising providing at least onecomestible or medicinal product, or a precursor thereof, and combiningthe comestible or medicinal product or precursor thereof with at least asavory flavor inhibiting amount of at least one non-naturally occurringcompound as disclosed herein, or a comestibly acceptable salt thereof,so as to form a modified comestible or medicinal product; therebyinhibiting the savory taste of a comestible or medicinal product.

Also disclosed are methods for increasing the savory taste of acomestible or medicinal product comprising providing at least onecomestible or medicinal product, or a precursor thereof, and combiningthe comestible or medicinal product or precursor thereof with at least asavory flavor increasing amount of at least one non-naturally occurringcompound as disclosed herein, or a comestibly acceptable salt thereof,so as to form a modified comestible or medicinal product; therebyincreasing the savory taste of a comestible or medicinal product.

Also disclosed are methods for modulating the sweet taste of acomestible or medicinal product comprising providing at least onecomestible or medicinal product, or a precursor thereof, and combiningthe comestible or medicinal product or precursor thereof with at least asweet flavor modulating amount of at least one non-naturally occurringcompound as disclosed herein, or a comestibly acceptable salt thereof,so as to form a modified comestible or medicinal product; therebymodulating the sweet taste of a comestible or medicinal product.

Also disclosed are methods for inhibiting the sweet taste of acomestible or medicinal product comprising providing at least onecomestible or medicinal product, or a precursor thereof, and combiningthe comestible or medicinal product or precursor thereof with at least asweet flavor inhibiting amount of at least one non-naturally occurringcompound as disclosed herein, or a comestibly acceptable salt thereof,so as to form a modified comestible or medicinal product; therebyinhibiting the sweet taste of a comestible or medicinal product.

Also disclosed are methods for increasing the sweet taste of acomestible or medicinal product comprising providing at least onecomestible or medicinal product, or a precursor thereof, and combiningthe comestible or medicinal product or precursor thereof with at least asweet flavor increasing amount of at least one non-naturally occurringcompound as disclosed herein, or a comestibly acceptable salt thereof,so as to form a modified comestible or medicinal product; therebyincreasing the sweet taste of a comestible or medicinal product.

Also disclosed are methods of enhancing umami taste perceptioncomprising contacting an umami receptor with cyclamate and NHDC, andtheir derivatives, as well as methods of enhancing umami tasteperception comprising contacting an umami receptor with lactisolederivatives. Also disclosed are methods of enhancing sweet tasteperception comprising contacting an sweet receptor with cyclamate andNHDC, and their derivatives. Also disclosed are methods of enhancingsweet taste perception comprising contacting an sweet receptor withlactisole derivatives.

Isolation and Expression of T1R Polypeptides

Isolation and expression of the T1Rs, or fragments or variants thereof,of the invention can be performed as described below. PCR primers can beused for the amplification of nucleic acids encoding taste receptorligand-binding regions, and libraries of these nucleic acids canoptionally be generated. Individual expression vectors or libraries ofexpression vectors can then be used to infect or transfect host cellsfor the functional expression of these nucleic acids or libraries. Thesegenes and vectors can be made and expressed in vitro or in vivo. One ofskill will recognize that desired phenotypes for altering andcontrolling nucleic acid expression can be obtained by modulating theexpression or activity of the genes and nucleic acids (e.g., promoters,enhancers and the like) within the vectors of the invention. Any of theknown methods described for increasing or decreasing expression oractivity can be used. The invention can be practiced in conjunction withany method or protocol known in the art, which are well described in thescientific and patent literature.

The nucleic acid sequences of the invention and other nucleic acids usedto practice this invention, whether RNA, cDNA, genomic DNA, vectors,viruses or hybrids thereof, may be isolated from a variety of sources,genetically engineered, amplified, and/or expressed recombinantly. Anyrecombinant expression system can be used, including, in addition tomammalian cells, e.g., bacterial, yeast, insect, or plant systems.

Alternatively, these nucleic, acids can be synthesized in vitro bywell-known chemical synthesis techniques, as described in, e.g.,Carruthers, Cold Spring Harbor Symp. Quant. Biol. 47:411-418 (1982);Adams, Am. Chem. Soc. 105:661 (1983); Belousov, Nucleic Acids Res.25:3440-3444 (1997); Frenkel, Free Radic. Biol. Med. 19:373-380 (1995);Blommers, Biochemistry 33:7886-7896 (1994); Narang, Meth. Enzymol. 68:90(1979); Brown, Meth. Enzymol. 68:109 (1979); Beaucage, Tetra. Lett.22:1859 (1981); U.S. Pat. No. 4,458,066. Double-stranded DNA fragmentsmay then be obtained either by synthesizing the complementary strand andannealing the strands together under appropriate conditions, or byadding the complementary strand using DNA polymerase with an appropriateprimer sequence.

Techniques for the manipulation of nucleic acids, such as, for example,for generating mutations in sequences, subcloning, labeling probes,sequencing, hybridization and the like are well described in thescientific and patent literature. See, e.g., Sambrook, ed., MolecularCloning: a Laboratory manual (2nd ed.), Vols. 1-3, Cold Spring HarborLaboratory (1989); Current Protocols in Molecular, Biology, Ausubel, ed.John Wiley & Sons, Inc., New York (1997); Laboratory Techniques inBiochemistry and Molecular Biology: Hybridization With Nucleic AcidProbes, Part I, Theory and Nucleic Acid Preparation, Tijssen, ed.Elsevier, N.Y. (1993).

Nucleic acids, vectors, capsids, polypeptides, and the like can beanalyzed and quantified by any of a number of general means well knownto those of skill in the art. These include, e.g., analyticalbiochemical methods such as NMR, spectrophotometry, radiography,electrophoresis, capillary electrophoresis, high performance liquidchromatography (HPLC), thin layer chromatography (TLC), andhyperdiffusion chromatography, various immunological methods, e.g.,fluid or gel precipitin reactions, immunodiffusion,immunoelectrophoresis, radioimmunoassays (RIAs), enzyme-linkedimmunosorbent assays (ELISAs), immuno-fluorescent assays, Southernanalysis, Northern analysis, dot-blot analysis, gel electrophoresis(e.g., SDS-PAGE), RT-PCR, quantitative PCR, other nucleic acid or targetor signal amplification methods, radiolabeling, scintillation counting,and affinity chromatography.

Oligonucleotide primers may be used to amplify nucleic acid fragmentsencoding taste receptor ligand-binding regions. The nucleic acidsdescribed herein can also be cloned or measured quantitatively usingamplification techniques. Amplification methods are also well known inthe art, and include, e.g., polymerase chain reaction, PCR (PCRProtocols, a Guide to Methods and Applications, ed. Innis. AcademicPress, N.Y. (1990) and PCR Strategies, ed. Innis, Academic Press, Inc.,N.Y. (1995), ligase chain reaction (LCR) (see, e.g., Wu, Genomics 4:560(1989); Landegren, Science 241:1077, (1988); Barringer, Gene 89:117(1990)); transcription amplification (see, e.g., Kwoh, Proc. Natl. Acad.Sci. USA 86:1173 (1989)); and, self-sustained sequence replication (see,e.g., Guatelli, Proc. Natl. Acad. Sci. USA 87:1874 (1990)); Q Betareplicase amplification (see, e.g., Smith, J. Clin. Microbiol.35:1477-1491 (1997)); automated Q-beta replicase amplification assay(see, e.g., Burg, Mol. Cell. Probes 10:257-271 (1996)) and other RNApolymerase mediated techniques (e.g., NASBA, Cangene, Mississauga,Ontario); see also Berger, Methods Enzymol. 152:307-316 (1987);Sambrook; Ausubel; U.S. Pat. Nos. 4,683,195 and 4,683,202; Sooknanan,Biotechnology 13:563-564 (1995). The primers can be designed to retainthe original sequence of the “donor” 7-membrane receptor. Alternatively,the primers can encode amino acid residues that are conservativesubstitutions (e.g., hydrophobic for hydrophobic residue, see abovediscussion) or functionally benign substitutions (e.g., do not preventplasma membrane insertion, cause cleavage by peptidase, cause abnormalfolding of receptor, and the like). Once amplified, the nucleic acids,either individually or as libraries, may be cloned according to methodsknown in the art, if desired, into any of a variety of vectors usingroutine molecular biological methods; methods for cloning in vitroamplified nucleic acids are described, e.g., U.S. Pat. No. 5,426,039.

The primer pairs may be designed to selectively amplify ligand-bindingregions of the T1R family members. These regions may vary for differentligands or tastants. Thus, what may be a minimal binding region for onetastant, may be too limiting for a second tastant. Accordingly,ligand-binding regions of different sizes comprising differentextracellular domain structures may be amplified.

Paradigms to design degenerate primer pairs are well known in the art.For example, a COnsensus-DEgenerate Hybrid Oligonucleotide Primer(CODEHOP) strategy computer program is accessible ashttp://blocks.fhcrc.org/codehop.html, and is directly linked from theBlockMaker multiple sequence alignment site for hybrid primer predictionbeginning with a set of related protein sequences, as known tastereceptor ligand-binding regions (see, e.g., Rose, Nucleic Acids Res.26:1628-1635 (1998); Singh, Biotechniques 24:318-319 (1998)).

Means to synthesize oligonucleotide primer pairs are well known in theart. “Natural” base pairs or synthetic base pairs can be used. Forexample, use of artificial nucleobases offers a versatile approach tomanipulate primer sequence and generate a more complex mixture ofamplification products. Various families of artificial nucleobases arecapable of assuming multiple hydrogen bonding orientations throughinternal bond rotations to provide a means for degenerate molecularrecognition. Incorporation of these analogs into a single position of aPCR primer allows for generation of a complex library of amplificationproducts. See, e.g., Hoops, Nucleic Acids Res. 25:4866-4871 (1997).Nonpolar molecules can also be used to mimic the shape of natural DNAbases. A non-hydrogen-bonding shape mimic for adenine can replicateefficiently and selectively against a nonpolar shape mimic for thymine(see, e.g., Morales, Nat. Struct. Biol. 5:950-954 (1998)). For example,two degenerate bases can be the pyrimidine base 6H,8H-3,4-dihydropyrimido[4,5-c][1,2]oxazin-7-one or the purine baseN6-methoxy-2,6-diaminopurine (see, e.g., Hill, Proc. Natl. Acad. Sci.USA 95:4258-4263 (1998)). Exemplary degenerate primers of the inventionincorporate the nucleobase analog5′-Dimethoxytrityl-N-benzoyl-2′-deoxy-Cytidine,3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite(the term “P” in the sequences, see above). This pyrimidine analoghydrogen bonds with purines, including A and G residues.

Polymorphic variants, alleles, and interspecies homologs that aresubstantially identical to a taste receptor disclosed herein can beisolated using the nucleic acid probes described above. Alternatively,expression libraries can be used to clone T1R polypeptides andpolymorphic variants, alleles, and interspecies homologs thereof, bydetecting expressed homologs immunologically with antisera or purifiedantibodies made against a T1R polypeptide, which also recognize andselectively bind to the T1R homolog.

Nucleic acids that encode ligand-binding regions of taste receptors maybe generated by amplification (e.g., PCR) of appropriate nucleic acidsequences using degenerate primer pairs. The amplified nucleic acid canbe genomic DNA from any cell or tissue or mRNA or cDNA derived fromtaste receptor-expressing cells.

In one embodiment, hybrid protein-coding sequences comprising nucleicacids encoding T1Rs fused to translocation sequences may be constructed.Also provided are hybrid T1Rs comprising the translocation motifs andtastant-binding domains of other families of chemosensory receptors,particularly taste receptors. These nucleic acid sequences can beoperably linked to transcriptional or translational control elements,e.g., transcription and translation initiation sequences, promoters andenhancers, transcription and translation terminators, polyadenylationsequences, and other sequences useful for transcribing DNA into RNA. Inconstitutive of recombinant expression cassettes, vectors, andtransgenics, a promoter fragment can be employed to direct expression ofthe desired nucleic acid in all desired cells or tissues.

In another embodiment, fusion proteins may include C-terminal orN-terminal translocation sequences. Further, fusion proteins cancomprise additional elements, e.g., for protein detection, purification,or other applications. Detection and purification facilitating domainsinclude, e.g., metal chelating peptides such as polyhistidine tracts,histidine-tryptophan modules, or other domains that allow purificationon immobilized metals; maltose binding protein; protein A domains thatallow purification on immobilized immunoglobulin; or the domain utilizedin the FLAGS extension/affinity purification system (Immunex Corp.,Seattle, Wash.).

The inclusion of a cleavable linker sequences such as Factor Xa (see,e.g., Ottavi, Biochimie 80:289-293 (1998)), subtilisin proteaserecognition motif (see, e.g., Polyak, Protein Eng. 10:615-619 (1997));enterokinase (Invitrogen, San Diego, Calif.), and the like, between thetranslocation domain (for efficient plasma membrane expression) and therest of the newly translated polypeptide may be useful to facilitatepurification. For example, one construct can include a polypeptideencoding a nucleic acid sequence linked to six histidine residuesfollowed by a thioredoxin, an enterokinase cleavage site (see, e.g.,Williams, Biochemistry 34:1787-1797 (1995)), and an C-terminaltranslocation domain. The histidine residues facilitate detection andpurification while the enterokinase cleavage site provides a means forpurifying the desired protein(s) from the remainder of the fusionprotein. Technology pertaining to vectors encoding fusion proteins andapplication of fusion proteins are well described in the scientific andpatent literature, see, e.g., Kroll, DNA Cell. Biol. 12:441-53 (1993).

Expression vectors, either as individual expression vectors or aslibraries of expression vectors, comprising the ligand-binding domainencoding sequences may be introduced into a genome or into the cytoplasmor a nucleus of a cell and expressed by a variety of conventionaltechniques, well described in the scientific and patent literature. See,e.g., Roberts, Nature 328:731 (1987); Berger supra; Schneider, ProteinExpr. Purif. 6435:10 (1995); Sambrook; Tijssen; Ausubel. Productinformation from manufacturers of biological reagents and experimentalequipment also provide information regarding known biological methods.The vectors can be isolated from natural sources, obtained from suchsources as ATCC or GenBank libraries, or prepared by synthetic orrecombinant methods.

The nucleic acids can be expressed using expression cassettes, vectorsor viruses which are stably or transiently expressed in cells (e.g.,episomal expression systems). Selection markers can be incorporated intoexpression cassettes and vectors to confer a selectable phenotype ontransformed cells and sequences. For example, selection markers can codefor episomal maintenance and replication such that integration into thehost genome is not required. For example, the marker may encodeantibiotic resistance (e.g., chloramphenicol, kanamycin, G418,blasticidin, hygromycin) or herbicide resistance (e.g., chlorosulfuronor Basta) to permit selection of those cells transformed with thedesired DNA sequences (see, e.g., Blondelet-Rouault, Gene 190:315-317(1997); Aubrecht, J. Pharmacol. Exp. Ther. 281:992-997 (1997)). Becauseselectable marker genes conferring resistance to substrates likeneomycin or hygromycin can only be utilized in tissue culture,chemoresistance genes are also used as selectable markers in vitro andin vivo.

A chimeric nucleic acid sequence may encode a T1R ligand-binding domainwithin any 7-transmembrane polypeptide. Because 7-transmembrane receptorpolypeptides have similar primary sequences and secondary and tertiarystructures, structural domains (e.g., extracellular domain, TM domains,cytoplasmic domain, etc.) can be readily identified by sequenceanalysis. For example, homology modeling, Fourier analysis and helicalperiodicity detection can identify and characterize the seven domainswith a 7-transmembrane receptor sequence. Fast Fourier Transform (FFT)algorithms can be used to assess the dominant periods that characterizeprofiles of the hydrophobicity and variability of analyzed sequences.Periodicity detection enhancement and alpha helical periodicity indexcan be done as by, e.g., Donnelly, Protein Sci. 2:55-70 (1993). Otheralignment and modeling algorithms are well known in the art, see, e.g.,Peitsch, Receptors Channels 4:161-164 (1996); Kyte & Doolittle, J. Med.Bio., 157:105-132 (1982); Cronet, Protein Eng. 6:59-64 (1993).

The present invention also includes not only the DNA and proteins havingthe specified nucleic and amino acid sequences, but also DNA fragments,particularly fragments of, e.g., 40, 60, 80, 100, 150, 200, or 250nucleotides, or more, as well as protein fragments of, e.g., 10, 20, 30,50, 70, 100, or 150 amino acids, or more. Optionally, the nucleic acidfragments can encode an antigenic polypeptide, which is capable ofbinding to an antibody raised against a T1R family member. Further, aprotein fragment of the invention can optionally be an antigenicfragment, which is capable of binding to an antibody raised against aT1R family member.

Also contemplated are chimeric proteins, comprising at least 10, 20, 30,50, 70, 100, or 150 amino acids, or more, of one of at least one of theT1R polypeptides described herein, coupled to additional amino acidsrepresenting all or part of another GPCR, preferably a member of the 7transmembrane superfamily. These chimeras can be made from the instantreceptors and another GPCR, or they can be made by combining two or moreof the present T1R receptors. In one embodiment, one portion of thechimera corresponds to or is derived from the extracellular domain of aT1R polypeptide of the invention. In another embodiment, one portion ofthe chimera corresponds to, or is derived from the extracellular domainand one or more of the transmembrane domains of a T1R polypeptidedescribed herein, and the remaining portion or portions can come fromanother GPCR. Chimeric receptors are well known in the art, and thetechniques for creating them and the selection and boundaries of domainsor fragments of G protein-coupled receptors for incorporation thereinare also well known. Thus, this knowledge of those skilled in the artcan readily be used to create such chimeric receptors. The use of suchchimeric receptors can provide, for example, a taste selectivitycharacteristic of one of the receptors specifically disclosed herein,coupled with the signal transduction characteristics of anotherreceptor, such as a well known receptor used in prior art assay systems.

As noted above, such chimeras, analogous to the native T1R receptor, ornative T1R receptor combination or association will bind to and/or beactivated by molecules that normally affect sweet taste or umami taste.Functional chimeric T1R receptors or receptor combinations are moleculeswhich when expressed alone or in combination with other T1Rs or otherGPCRs (which may themselves be chimeric) bind to or which are activatedby taste stimuli, particularly sweet (T1R2/3) or umami taste stimuli(T1R1/3). Molecules that elicit sweet taste include natural andartificial sweeteners such as sucrose, aspartame, xylitol, cyclamate, etal., Molecules that elicit umami taste include glutamate and glutamateanalogs and other compounds that bind to native T1R1 and/or T1R3, suchas 5′-nucleotides.

For example, a domain such as a ligand-binding domain, an extracellulardomain, a transmembrane domain, a transmembrane domain, a cytoplasmicdomain, an N-terminal domain, a C-terminal domain, or any combinationthereof, can be covalently linked to a heterologous protein. Forinstance, an T1R extracellular domain can be linked to a heterologousGPCR transmembrane domain, or a heterologous GPCR extracellular domaincan be linked to a T1R transmembrane domain. Other heterologous proteinsof choice can be used; e.g., green fluorescent protein.

Also within the scope of the invention are host cells for expressing theT1Rs, fragments, chimeras or variants of the invention. To obtain highlevels of expression of a cloned gene or nucleic acid, such as cDNAsencoding the T1Rs, fragments, or variants of the invention, one of skilltypically subclones the nucleic acid sequence of interest into anexpression vector that contains a strong promoter to directtranscription, a transcription/translation terminator, and if for anucleic acid encoding a protein, a ribosome binding site fortranslational initiation. Suitable bacterial promoters are well known inthe art and described, e.g., in Sambrook et al. However, bacterial oreukaryotic expression systems can be used.

Any of the well-known procedures for introducing foreign nucleotidesequences into host cells may be used. These include the use of calciumphosphate transfection, polybrene, protoplast fusion, electroporation,liposomes, microinjection, plasma vectors, viral vectors and any of theother well known methods for introducing cloned genomic DNA, cDNA,synthetic DNA or other foreign genetic material into a host cell (see,e.g., Sambrook et al.) It is only necessary that the particular geneticengineering procedure used be capable of successfully introducing atlest one nucleic acid molecule into the host cell capable of expressingthe T1R, fragment, or variant of interest.

After the expression vector is introduced into the cells, thetransfected cells are cultured under conditions favoring expression ofthe receptor, fragment, or variant of interest, which is then recoveredfrom the culture using standard techniques. Examples of such techniquesare well known in the art. See, e.g., WO 00/06593, which is incorporatedby reference in a manner consistent with this disclosure.

Detection of T1R Polypeptides

In addition to the detection of T1R genes and gene expression usingnucleic acid hybridization technology, one can also use immunoassays todetect T1Rs, e.g., to identify taste receptor cells, and variants of T1Rfamily members. Immunoassays can be used to qualitatively orquantitatively analyze the T1Rs. A general overview of the applicabletechnology can be found in Harlow & Lane, Antibodies: A LaboratoryManual (1988).

1. Antibodies to T1R Family Members

Methods of producing polyclonal and monoclonal antibodies that reactspecifically with a T1R family member are known to those of skill in theart (see, e.g., Coligan, Current Protocols in Immunology (1991); Harlow& Lane, supra; Goding, Monoclonal Antibodies: Principles and Practice(2d ed. 1986); and Kohler & Milstein, Nature, 256:495-497 (1975)). Suchtechniques include antibody preparation by selection of antibodies fromlibraries of recombinant antibodies in phage or similar vectors, as wellas preparation of polyclonal and monoclonal antibodies by immunizingrabbits or mice (see, e.g., Huse et al., Science, 246:1275-1281 (1989);Ward et al., Nature, 341:544-546 (1989)).

A number of T1R-comprising immunogens may be used to produce antibodiesspecifically reactive with a T1R family member. For example, arecombinant T1R polypeptide, or an antigenic fragment thereof, can beisolated as described herein. Suitable antigenic regions include, e.g.,the consensus sequences that are used to identify members of the T1Rfamily. Recombinant proteins can be expressed in eukaryotic orprokaryotic cells as described above, and purified as generallydescribed above. Recombinant protein is the preferred immunogen for theproduction of monoclonal or polyclonal antibodies. Alternatively, asynthetic peptide derived from the sequences disclosed herein andconjugated to a carrier protein can be used an immunogen. Naturallyoccurring protein may also be used either in pure or impure form. Theproduct is then injected into an animal capable of producing antibodies.Either monoclonal or polyclonal antibodies may be generated, forsubsequent use in immunoassays to measure the protein.

Methods of production of polyclonal antibodies are known to those ofskill in the art. For example, an inbred strain of mice (e.g., BALB/Cmice) or rabbits is immunized with the protein using a standardadjuvant, such as Freund's adjuvant, and a standard immunizationprotocol. The animal's immune response to the immunogen preparation ismonitored by taking test bleeds and determining the titer of reactivityto the T1R. When appropriately high titers of antibody to the immunogenare obtained, blood is collected from the animal and antisera areprepared. Further fractionation of the antisera to enrich for antibodiesreactive to the protein can be done if desired (see Harlow & Lane,supra).

Monoclonal antibodies may be obtained by various techniques familiar tothose skilled in the art. Briefly, spleen cells from an animal immunizedwith a desired antigen may be immortalized, commonly by fusion with amyeloma cell (see Kohler & Milstein, Eur. J. Immunol., 6:511-519(1976)). Alternative methods of immortalization include transformationwith Epstein Barr Virus, oncogenes, or retroviruses, or other methodswell known in the art. Colonies arising from single immortalized cellsare screened for production of antibodies of the desired specificity andaffinity for the antigen, and yield of the monoclonal antibodiesproduced by such cells may be enhanced by various techniques, includinginjection into the peritoneal cavity of a vertebrate host.Alternatively, one may isolate DNA sequences which encode a monoclonalantibody or a binding fragment thereof by screening a DNA library fromhuman B cells according to the general protocol outlined by Huse et al.,Science, 246:1275-1281 (1989).

Monoclonal antibodies and polyclonal sera are collected and titeredagainst the immunogen protein in an immunoassay, for example, a solidphase immunoassay with the immunogen immobilized on a solid support.Typically, polyclonal antisera with a titer of 104 or greater areselected and tested for their cross reactivity against non-T1Rpolypeptides, or even other T1R family members or other related proteinsfrom other organisms, using a competitive binding immunoassay. Specificpolyclonal antisera and monoclonal antibodies will usually bind with aKd of at least about 0.1 mM, more usually at least about 1 pM,optionally at least about 0.1 pM or better, and optionally 0.01 pM orbetter.

Once T1R family member specific antibodies are available, individual T1Rproteins and protein fragments can be detected by a variety ofimmunoassay methods. For a review of immunological and immunoassayprocedures, see Basic and Clinical Immunology (Stites & Terr eds., 7thed. 1991). Moreover, the immunoassays of the present invention can beperformed in any of several configurations, which are reviewedextensively in Enzyme Immunoassay (Maggio, ed., 1980); and Harlow &Lane, supra.

2. Immunological Binding Assays

T1R proteins, fragments, and variants can be detected and/or quantifiedusing any of a number of well-recognized immunological binding assays(see, e.g., U.S. Pat. Nos. 4,366,241; 4,376,110; 4,517,288; and4,837,168). For a review of the general immunoassays, see also Methodsin Cell Biology: Antibodies in Cell Biology, volume 37 (Asai, ed. 1993);Basic and Clinical Immunology (Stites & Terr, eds., 7th ed. 1991).Immunological binding assays (or immunoassays) typically use an antibodythat specifically binds to a protein or antigen of choice (in this casea T1R family member or an antigenic subsequence thereof). The antibody(e.g., anti-T1R) may be produced by any of a number of means well knownto those of skill in the art and as described above.

Immunoassays also often use a labeling agent to specifically bind to andlabel the complex formed by the antibody and antigen. The labeling agentmay itself be one of the moieties comprising the antibody/antigencomplex. Thus, the labeling agent may be a labeled T1R polypeptide or alabeled anti-T1R antibody. Alternatively, the labeling agent may be athird moiety, such a secondary antibody that specifically binds to theantibody/T1R complex (a secondary antibody is typically specific toantibodies of the species from which the first antibody is derived).Other proteins capable of specifically binding immunoglobulin constantregions, such as protein A or protein G may also be used as the labelagent. These proteins exhibit a strong non-immunogenic reactivity withimmunoglobulin constant regions from a variety of species (see, e.g.,Kronval et al., J. Immunol., 111: 1401-1406 (1973); Akerstrom et al., J.Immunol., 135:2589-2542 (1985)). The labeling agent can be modified witha detectable moiety, such as biotin, to which another molecule canspecifically bind, such as streptavidin. A variety of detectablemoieties are well known to those skilled in the art.

Throughout the assays, incubation and/or washing steps may be requiredafter each combination of reagents. Incubation steps can vary from about5 seconds to several hours, optionally from about 5 minutes to about 24hours. However, the incubation time will depend upon the assay format,antigen, volume of solution, concentrations, and the like. Usually, theassays will be carried out at ambient temperature, although they can beconducted over a range of temperatures, such as 10° C. to 40° C.

A. Non-Competitive Assay Formats

Immunoassays for detecting a T1R polypeptide in a sample may be eithercompetitive or noncompetitive. Noncompetitive immunoassays are assays inwhich the amount of antigen is directly measured. In one preferred“sandwich” assay, for example, the anti-T1R antibodies can be bounddirectly to a solid substrate on which they are immobilized. Theseimmobilized antibodies then capture the T1R polypeptide present in thetest sample. The T1R polypeptide is thus immobilized is then bound by alabeling agent, such as a second T1R antibody bearing a label.Alternatively, the second antibody may lack a label, but it may, inturn, be bound by a labeled third antibody specific to antibodies of thespecies from which the second antibody is derived. The second or thirdantibody is typically modified with a detectable moiety, such as biotin,to which another molecule specifically binds, e.g., streptavidin, toprovide a detectable moiety.

B. Competitive Assay Formats

In competitive assays, the amount of T1R polypeptide present in thesample is measured indirectly by measuring the amount of a known, added(exogenous) T1R polypeptide displaced (competed away) from an anti-T1Rantibody by the unknown T1R polypeptide present in a sample. In onecompetitive assay, a known amount of T1R polypeptide is added to asample and the sample is then contacted with an antibody thatspecifically binds to the T1R. The amount of exogenous T1R polypeptidebound to the antibody is inversely proportional to the concentration ofT1R polypeptide present in the sample. In a particularly preferredembodiment, the antibody is immobilized on a solid substrate. The amountof T1R polypeptide bound to the antibody may be determined either bymeasuring the amount of T1R polypeptide present in a T1R/antibodycomplex, or alternatively by measuring the amount of remaininguncomplexed protein. The amount of T1R polypeptide may be detected byproviding a labeled T1R molecule.

A hapten inhibition assay is another preferred competitive assay. Inthis assay the known T1R polypeptide is immobilized on a solidsubstrate. A known amount of anti-T1R antibody is added to the sample,and the sample is then contacted with the immobilized T1R. The amount ofanti-T1R antibody bound to the known immobilized T1R polypeptide isinversely proportional to the amount of T1R polypeptide present in thesample. Again, the amount of immobilized antibody may be detected bydetecting either the immobilized fraction of antibody or the fraction ofthe antibody that remains in solution. Detection may be direct where theantibody is labeled or indirect by the subsequent addition of a labeledmoiety that specifically binds to the antibody as described above.

C. Cross-Reactivity Determinations

Immunoassays in the competitive binding format can also be used forcross-reactivity determinations. For example, a protein at leastpartially encoded by the nucleic acid sequences disclosed herein can beimmobilized to a solid support. Proteins (e.g., T1R polypeptides andhomologs) are added to the assay that compete for binding of theantisera to the immobilized antigen. The ability of the added proteinsto compete for binding of the antisera to the immobilized protein iscompared to the ability of the T1R polypeptide encoded by the nucleicacid sequences disclosed herein to compete with itself. The percentcross-reactivity for the above proteins is calculated, using standardcalculations. Those antisera with less than 10% cross-reactivity witheach of the added proteins listed above are selected and pooled. Thecross-reacting antibodies are optionally removed from the pooledantisera by immunoabsorption with the added considered proteins, e.g.,distantly related homologs. In addition, peptides comprising amino acidsequences representing conserved motifs that are used to identifymembers of the T1R family can be used in cross-reactivitydeterminations.

The immunoabsorbed and pooled antisera are then used in a competitivebinding immunoassay as described above to compare a second protein,thought to be perhaps an allele or polymorphic variant of a T1R familymember, to the immunogen protein (i.e., T1R polypeptide encoded by thenucleic acid sequences disclosed herein). In order to make thiscomparison, the two proteins are each assayed at a wide range ofconcentrations and the amount of each protein required to inhibit 50% ofthe binding of the antisera to the immobilized protein is determined. Ifthe amount of the second protein required to inhibit 50% of binding isless than 10 times the amount of the protein encoded by nucleic acidsequences disclosed herein required to inhibit 50% of binding, then thesecond protein is said to specifically bind to the polyclonal antibodiesgenerated to a T1R immunogen.

Antibodies raised against T1R conserved motifs can also be used toprepare antibodies that specifically bind only to GPCRs of the T1Rfamily, but not to GPCRs from other families.

Polyclonal antibodies that specifically bind to a particular member ofthe T1R family can be made by subtracting out cross-reactive antibodiesusing other T1R family members. Species-specific polyclonal antibodiescan be made in a similar way. For example, antibodies specific to humanT1R1 can be made by, subtracting out antibodies that are cross-reactivewith orthologous sequences, e.g., rat T1R1 or mouse T1R1.

D. Other Assay Formats

Western blot (immunoblot) analysis is used to detect and quantify thepresence of T1R polypeptide in the sample. The technique generallycomprises separating sample proteins by gel electrophoresis on the basisof molecular weight, transferring the separated proteins to a suitablesolid support, (such as a nitrocellulose filter, a nylon filter, orderivatized nylon filter), and incubating the sample with the antibodiesthat specifically bind the T1R polypeptide. The anti-T1R polypeptideantibodies specifically bind to the T1R polypeptide on the solidsupport. These antibodies may be directly labeled or alternatively maybe subsequently detected using labeled antibodies (e.g., labeled sheepanti-mouse antibodies) that specifically bind to the anti-T1Rantibodies.

Other, assay formats include liposome immunoassays (LIA), which useliposomes designed to bind specific molecules (e.g., antibodies) andrelease encapsulated reagents or markers. The released chemicals arethen detected according to standard techniques (see Monroe et al., Amer.Clin. Prod. Rev., 5:34-41 (1986)).

E. Reduction of Non-Specific Binding

One of skill in the art will appreciate that it is often desirable tominimize non-specific binding in immunoassays. Particularly, where theassay involves an antigen or antibody immobilized on a solid substrateit is desirable to minimize the amount of non-specific binding to thesubstrate. Means of reducing such non-specific binding are well known tothose of skill in the art. Typically, this technique involves coatingthe substrate with a proteinaceous composition. In particular, proteincompositions such as bovine serum albumin (BSA), nonfat powdered milk,and gelatin are widely used with powdered milk being most preferred.

F. Labels

The particular label or detectable group used in the assay is not acritical aspect of the invention, as long as it does not significantlyinterfere with the specific binding of the antibody used in the assay.The detectable group can be any material having a detectable physical orchemical property. Such detectable labels have been well developed inthe field of immunoassays and, in general, most any label useful in suchmethods can be applied to the present invention. Thus, a label is anycomposition detectable by spectroscopic, photochemical, biochemical,immunochemical, electrical, optical, or chemical means. Useful labels inthe present invention include magnetic beads (e.g., DYNABEADS™),fluorescent dyes (e.g., fluorescein isothiocyanate, Texas red,rhodamine, and the like), radiolabels (e.g., ³H, ¹²⁵I, ¹⁴C, ³⁵S),enzymes (e.g., horseradish peroxidase, alkaline phosphates and otherscommonly used in an ELISA), and colorimetric labels such as colloidalgold or colored glass or plastic beads (e.g., polystyrene,polypropylene, latex, etc.).

The label may be coupled directly or indirectly to the desired componentof the assay according to methods well known in the art. As indicatedabove, a wide variety of labels may be used, with the choice of labeldepending on sensitivity required, ease of conjugation with thecompound, stability requirements, available instrumentation, anddisposal provisions.

Non-radioactive labels are often attached by indirect means. Generally,a ligand molecule (e.g., biotin) is covalently bound to the molecule.The ligand then binds to another molecules (e.g., streptavidin)molecule, which is either inherently detectable or covalently bound to asignal system, such as a detectable enzyme, a fluorescent compound, or achemiluminescent compound. The ligands and their targets can be used inany suitable combination with antibodies that recognize a T1Rpolypeptide, or secondary antibodies that recognize anti-T1R.

The molecules can also be conjugated directly to signal generatingcompounds, e.g., by conjugation with an enzyme or fluorophore. Enzymesof interest as labels will primarily be hydrolases, particularlyphosphatases, esterases and glycosidases, or oxidotases, particularlyperoxidases. Fluorescent compounds include fluorescein and itsderivatives, rhodamine and its derivatives, dansyl, umbelliferone, etc.Chemiluminescent compounds include luciferin, and2,3-dihydrophthalazinediones, e.g., luminol. For a review of variouslabeling or signal producing systems that may be used, see U.S. Pat. No.4,391,904.

Means of detecting labels are well known to those of skill in the art.Thus, for example, where the label is a radioactive label means fordetection include a scintillation counter or photographic film as inautoradiography. Where the label is a fluorescent label, it may bedetected by exciting the fluorochrome with the appropriate wavelength oflight and detecting the resulting fluorescence. The fluorescence may bedetected visually, by means of photographic film, by the use ofelectronic detectors such as charge-coupled devices (CCDs) orphotomultipliers and the like. Similarly, enzymatic labels may bedetected by providing the appropriate substrates for the enzyme anddetecting the resulting reaction product. Finally simple calorimetriclabels may be detected simply by observing the color associated with thelabel. Thus, in various dipstick assays, conjugated gold often appearspink, while various conjugated beads appear the color of the bead.

Some assay formats do not require the use of labeled components. Forinstance, agglutination assays can be used to detect the presence of thetarget antibodies. In this case, antigen-coated particles areagglutinated by samples comprising the target antibodies. In thisformat, none of the components need be labeled and the presence of thetarget antibody is detected by simple visual inspection.

Detection of Modulators

Compositions and methods for determining whether a test compoundspecifically binds to a T1R receptor of the invention, both in vitro andin vivo, are described below. Many aspects of cell physiology can bemonitored to assess the effect of ligand binding to a T1R polypeptide ofthe invention. These assays may be performed on intact cells expressinga chemosensory receptor, on permeabilized cells, or on membranefractions produced by standard methods or in vitro de novo synthesizedproteins.

In vivo, taste receptors bind tastants and initiate the transduction ofchemical stimuli into electrical signals. An activated or inhibited Gprotein will in turn alter the properties of target enzymes, channels,and other effector proteins. Some examples are the activation of cGMPphosphodiesterase by transducin in the visual system, adenylate cyclaseby the stimulatory G protein, phospholipase C by Gq and other cognate Gproteins, and modulation of diverse channels by Gi and other G proteins.Downstream consequences can also be examined such as generation ofdiacyl glycerol and IP3 by phospholipase C, and in turn, for calciummobilization by IP3.

The T1R proteins or polypeptides of the assay will preferably beselected from a polypeptide having the T1R polypeptide sequence selectedfrom those disclosed in Example 1, or fragments or conservativelymodified variants thereof. Optionally, the fragments and variants can beantigenic fragments and variants which bind to an anti-T1R antibody.Optionally, the fragments and variants can bind to or are activated bysweeteners or umami tastants.

Alternatively, the T1R proteins or polypeptides of the assay can bederived from a eukaryotic host cell and can include an amino acidsubsequence having amino acid sequence identity to the T1R polypeptidesdisclosed in Example 1, or fragments or conservatively modified variantsthereof. Generally, the amino acid sequence identity will be at least 35to 50%, or optionally 75%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%.Optionally, the T1R proteins or polypeptides of the assays can comprisea domain of a T1R protein, such as an extracellular domain,transmembrane region, transmembrane domain, cytoplasmic domain,ligand-binding domain, and the like. Further, as described above, theT1R protein or a domain thereof can be covalently linked to aheterologous protein to create a chimeric protein used in the assaysdescribed herein.

Modulators of T1R receptor activity are tested using T1R proteins orpolypeptides as described above, either recombinant or naturallyoccurring. The T1R proteins or polypeptides can be isolated,co-expressed in a cell, co-expressed in a membrane derived from a cell,co-expressed in tissue or in an animal, either recombinant or naturallyoccurring. For example, tongue slices, dissociated cells from a tongue,transformed cells, or membranes can be used. Modulation can be testedusing one of the in vitro or in vivo assays described herein.

For example, as disclosed in the experiment examples infra, it has beendiscovered that certain 5′ nucleotides, e.g., 5′ IMP or 5′ GMP, enhancethe activity of L-glutamate to activate the umami taste receptor, orblock the activation of the umami taste receptor by umami taste stimulisuch as L-glutamate and L-aspartate.

1. In Vitro Binding Assays

Taste transduction can also be examined in vitro with soluble or solidstate reactions, using the T1R polypeptides of the invention. In aparticular embodiment, T1R ligand-binding domains can be used in vitroin soluble or solid state reactions to assay for ligand binding.

For instance, the T1R N-terminal domain is predicted to be involved inligand binding. More particularly, the T1Rs belong to a GPCR sub-familythat is characterized by large, approximately 600 amino acid,extracellular N-terminal segments. These N-terminal segments are thoughtto form the ligand-binding domains, and are therefore useful inbiochemical assays to identify T1R agonists and antagonists. It ispossible that the ligand-binding domain may be formed by additionalportions of the extracellular domain, such as the extracellular loops ofthe transmembrane domain.

In vitro binding assays have been used with other GPCRs that are relatedto the T1Rs, such as the metabotropic glutamate receptors (see, e.g.,Han and Hampson, J. Biol. Chem. 274:10008-10013 (1999)). These assaysmight involve displacing a radioactively or fluorescently labeledligand, measuring changes in intrinsic fluorescence or changes inproteolytic susceptibility, etc.

Ligand binding to a hetero-multimeric complex of T1R polypeptides of theinvention can be tested in solution, in a bilayer membrane, optionallyattached to a solid phase, in a lipid monolayer, or in vesicles. Bindingof a modulator can be tested using, e.g., changes in spectroscopiccharacteristics (e.g., fluorescence, absorbence, refractive index)hydrodynamic (e.g., shape), chromatographic, or solubility properties.

In another embodiment of the invention, a GTPγ³⁵S assay may be used. Asdescribed above, upon activation of a GPCR, the Gα subunit of the Gprotein complex is stimulated to exchange bound GDP for GTP.Ligand-mediated stimulation of G protein exchange activity can bemeasured in a biochemical assay measuring the binding of addedradioactively labeled GTPγ³⁵S to the G protein in the presence of aputative ligand. Typically, membranes containing the chemosensoryreceptor of interest are mixed with a complex of G proteins. Potentialinhibitors and/or activators and GTPγ³⁵S are added to the assay, andbinding of GTPγ³⁵S to the G protein is measured. Binding can be measuredby liquid scintillation counting or by any other means known in the art,including scintillation proximity assays (SPA). In other assays formats,fluorescently labeled GTPγS can be utilized.

2. Fluorescence Polarization Assays

In another embodiment, Fluorescence Polarization (“FP”) based assays maybe used to detect and monitor ligand binding. Fluorescence polarizationis a versatile laboratory technique for measuring equilibrium binding,nucleic acid hybridization, and enzymatic activity. Fluorescencepolarization assays are homogeneous in that they do not require aseparation step such as centrifugation, filtration, chromatography,precipitation, or electrophoresis. These assays are done in real time,directly in solution and do not require an immobilized phase.Polarization values can be measured repeatedly and after the addition ofreagents since measuring the polarization is rapid and does not destroythe sample. Generally, this technique can be used to measurepolarization values of fluorophores from low picomolar to micromolarlevels. This section describes how fluorescence polarization can be usedin a simple and quantitative way to measure the binding of ligands tothe T1R polypeptides of the invention.

When a fluorescently labeled molecule is excited with plane-polarizedlight, it emits light that has a degree of polarization that isinversely proportional to its molecular rotation. Large fluorescentlylabeled molecules remain relatively stationary during the excited state(4 nanoseconds in the case of fluorescein) and the polarization of thelight remains relatively constant between excitation and emission. Smallfluorescently labeled molecules rotate rapidly during the excited stateand the polarization changes significantly between excitation andemission. Therefore, small molecules have low polarization values andlarge molecules have high polarization values. For example, asingle-stranded fluorescein-labeled oligonucleotide has a relatively lowpolarization value but when it is hybridized to a complementary strand,it has a higher polarization value. When using FP to detect and monitortastant-binding which may activate or inhibit the chemosensory receptorsof the invention, fluorescence-labeled tastants or auto-fluorescenttastants may be used.

Fluorescence polarization (P) is defined as:

$P = \frac{{Int}_{\sqcup} - {Int}_{\bot}}{{Int}_{\sqcup} + {Int}_{\bot}}$Where Π is the intensity of the emission light parallel to theexcitation light plane and Int ⊥ is the intensity of the emission lightperpendicular to the excitation light plane. P, being a ratio of lightintensities, is a dimensionless number. For example, the Beacon® andBeacon 2000™ System may be used in connection with these assays. Suchsystems typically express polarization in millipolarization units (1Polarization Unit=1000 mP Units).

The relationship between molecular rotation and size is described by thePerrin equation and the reader is referred to Jolley, M. E. (1991) inJournal of Analytical Toxicology, pp. 236-240, which gives a thoroughexplanation of this equation. Summarily, the Perrin equation states thatpolarization is directly proportional to the rotational relaxation time,the time that it takes a molecule to rotate through an angle ofapproximately 68.5° Rotational relaxation time is related to viscosity(η), absolute temperature (T), molecular volume (V), and the gasconstant (R) by the following equation:

${{Rotational}\mspace{14mu}{Relaxation}\mspace{14mu}{Time}} = \frac{3\;\eta\; V}{RT}$

The rotational relaxation time is small (≈1 nanosecond) for smallmolecules (e.g. fluorescein) and large (≈100 nanoseconds) for largemolecules (e.g. immunoglobulins). If viscosity and temperature are heldconstant, rotational relaxation time, and therefore polarization, isdirectly related to the molecular volume. Changes in molecular volumemay be due to interactions with other molecules, dissociation,polymerization, degradation, hybridization, or conformational changes ofthe fluorescently labeled molecule. For example, fluorescencepolarization has been used to measure enzymatic cleavage of largefluorescein labeled polymers by proteases, DNases, and RNases. It alsohas been used to measure equilibrium binding for protein/proteininteractions, antibody/antigen binding, and protein/DNA binding.

A. Solid State and Soluble High Throughput Assays

In yet another embodiment, the invention provides soluble assays using ahetero-oligomeric T1R polypeptide complex; or a cell or tissueco-expressing T1R polypeptides. Preferably, the cell will comprise acell line that stably co-expresses a functional T1R1/T1R3 (umami) tastereceptor or T1R2/T1R3 (sweet) taste receptor. In another embodiment, theinvention provides solid phase based in vitro assays in a highthroughput format, where the T1R polypeptides, or cell or tissueexpressing the T1R polypeptides is attached to a solid phase substrateor a taste stimulating compound and contacted with a T1R receptor, andbinding detected using an appropriate tag or antibody raised against theT1R receptor.

In the high throughput assays of the invention, it is possible to screenup to several thousand-different modulators or ligands in a single day.In particular, each well of a microtiter plate can be used to run aseparate assay against a selected potential modulator, or, ifconcentration or incubation time effects are to be observed, every 5-10wells can test a single modulator. Thus, a single standard microtiterplate can assay about 100 (e.g., 96) modulators. If 1536 well plates areused, then a single plate can easily assay from about 1000 to about 1500different compounds. It is also possible to assay multiple compounds ineach plate well. It is possible to assay several different plates perday; assay screens for up to about 6,000-20,000 different compounds ispossible using the integrated systems of the invention. More recently,microfluidic approaches to reagent manipulation have been developed.

The molecule of interest can be bound to the solid state component,directly or indirectly, via covalent or non-covalent linkage, e.g., viaa tag. The tag can be any of a variety of components. In general, amolecule which binds the tag (a tag binder) is fixed to a solid support,and the tagged molecule of interest (e.g., the taste transductionmolecule of interest) is attached to the solid support by interaction ofthe tag and the tag binder.

A number of tags and tag binders can be used, based upon known molecularinteractions well described in the literature. For example, where a taghas a natural binder, for example, biotin, protein A, or protein G, itcan be used in conjunction with appropriate tag binders (avidin,streptavidin, neutravidin, the Fc region of an immunoglobulin, etc.).Antibodies to molecules with natural binders such as biotin are alsowidely available and appropriate tag binders (see, SIGMA Immunochemicals1998 catalogue SIGMA, St. Louis Mo.).

Similarly, any haptenic or antigenic compound can be used in combinationwith an appropriate antibody to form a tag/tag binder pair. Thousands ofspecific antibodies are commercially available and many additionalantibodies are described in the literature. For example, in one commonconfiguration, the tag is a first antibody and the tag binder is asecond antibody which recognizes the first antibody. In addition toantibody-antigen interactions, receptor-ligand interactions are alsoappropriate as tag and tag-binder pairs. For example, agonists andantagonists of cell membrane receptors (e.g. cell receptor-ligandinteractions such as transferrin, c-kit, viral receptor ligands,cytokine receptors, chemokine receptors, interleukin receptors,immunoglobulin receptors and antibodies, the cadherin family, theintegrin family, the selectin family, and the like; see, e.g., Pigott &Power, The Adhesion Molecule Facts Book I (1993)). Similarly, toxins andvenoms, viral epitopes, hormones (e.g., opiates, steroids, etc.),intracellular receptors (e.g., which mediate the effects of varioussmall ligands, including steroids, thyroid hormone, retinoids andvitamin D; peptides), drugs, lectins, sugars, nucleic acids (both linearand cyclic polymer configurations), oligosaccharides, proteins,phospholipids and antibodies can all interact with various cellreceptors.

Synthetic polymers, such as polyurethanes, polyesters, polycarbonates,polyureas, polyamides, polyethyleneimines, polyarylene sulfides,polysiloxanes, polyimides, and polyacetates can also form an appropriatetag or tag binder. Many other tag/tag binder pairs are also useful inassay systems described herein, as would be apparent to one of skillupon review of this disclosure.

Common linkers such as peptides, polyethers, and the like can also serveas tags, and include polypeptide sequences, such as poly gly sequencesof between about 5 and 200 amino acids. Such flexible linkers are knownto persons of skill in the art. For example, poly(ethylene glycol)linkers are available from Shearwater Polymers, Inc. Huntsville, Ala.These linkers optionally have amide linkages, sulfhydryl linkages, orheterofunctional linkages.

Tag binders are fixed to solid substrates using any of a variety ofmethods currently available. Solid substrates are commonly derivatizedor functionalized by exposing all or a portion of the substrate to achemical reagent which fixes a chemical group to the surface which isreactive with a portion of the tag binder. For example, groups which aresuitable for attachment to a longer chain portion would include amines,hydroxyl, thiol, and carboxyl groups. Aminoalkylsilanes andhydroxyalkylsilanes can be used to functionalize a variety of surfaces,such as glass surfaces. The constitutive of such solid phase biopolymerarrays is well described in the literature. See, e.g., Merrifield, J.Am. Chem. Soc., 85:2149-2154 (1963) (describing solid phase synthesisof, e.g., peptides); Geysen et al., J. Immun. Meth., 102:259-274 (1987)(describing synthesis of solid phase components on pins); Frank &Doring, Tetrahedron, 44:60316040 (1988) (describing synthesis of variouspeptide sequences on cellulose disks); Fodor et al., Science,251:767-777 (1991); Sheldon et al., Clinical Chemistry, 39(4):718-719(1993); and Kozal et al., Nature Medicine, 2(7):753759 (1996) (alldescribing arrays of biopolymers fixed to solid substrates).Non-chemical approaches for fixing tag binders to substrates includeother common methods, such as heat, cross-linking by UV radiation, andthe like.

3. Cell-Based Assays

In a preferred embodiment of treatment, a combination of T1R proteins orpolypeptides are transiently or stably co-expressed in a eukaryotic celleither in unmodified forms or as chimeric, variant or truncatedreceptors with or preferably without a heterologous, chaperone sequencethat facilitates its maturation and targeting through the secretorypathway. Such T1R polypeptides can be expressed in any eukaryotic cell,such as HEK-293 cells. Preferably, the cells comprise a functional Gprotein, e.g., Gα15 or the chimeric G protein previously identified, oranother G protein that is capable of coupling the chimeric receptor toan intracellular signaling pathway or to a signaling protein such asphospholipase C. Also, preferably a cell will be produced that stablyco-expresses T1R1/T1R3 or T1R2/T1R3 as such cells have been found (asshown in the experimental examples) to exhibit enhanced responses totaste stimuli (relation to cells that transiently express the same T1Rcombination). Activation of T1R receptors in such cells can be detectedusing any standard method, such as by detecting changes in intracellularcalcium by detecting Fluo-4 dependent fluorescence in the cell. Such anassay is the basis of the experimental findings presented in thisapplication.

Activated GPCR receptors often are substrates for kinases thatphosphorylate the C-terminal tail of the receptor (and possibly othersites as well). Thus, activators will promote the transfer of ³²P fromradiolabeled ATP to the receptor, which can be assayed with ascintillation counter. The phosphorylation of the C-terminal tail willpromote the binding of arrestin-like proteins and will interfere withthe binding of G proteins. For a general review of GPCR signaltransduction and methods of assaying signal transduction, see, e.g.,Methods in Enzymology, vols. 237 and 238 (1994) and volume 96 (1983);Bourne et al., Nature, 10:349:117-27 (1991); Bourne et al., Nature,348:125-32 (1990); Pitcher et al., Annu. Rev. Biochem., 67:653-92(1998).

T1R modulation may be assayed by comparing the response of T1Rpolypeptides treated with a putative T1R modulator to the response of anuntreated control sample or a sample containing a known “positive”control. Such putative T1R modulators can include molecules that eitherinhibit or activate T1R polypeptide activity. In one embodiment, controlsamples (untreated with activators or inhibitors) are assigned arelative T1R activity value of 100. Inhibition of a T1R polypeptide isachieved when the T1R activity value relative to the control is about90%, optionally 50%, optionally 25-0%. Activation of a T1R polypeptideis achieved when the T1R activity value relative to the control is 110%,optionally 150%, 200-500%, or 1000-2000%.

Changes in ion flux may be assessed by determining changes in ionicpolarization (i.e., electrical potential) of the cell or membraneexpressing a T1R polypeptide. One means to determine changes in cellularpolarization is by measuring changes in current (thereby measuringchanges in polarization) with voltage-clamp and patch-clamp techniques(see, e.g., the “cell-attached” mode, the “inside-out” mode, and the“whole cell” mode, e.g., Ackerman et al., New Engl. J Med.,336:1575-1595 (1997)). Whole cell currents are conveniently determinedusing the standard. Other known assays include: radiolabeled ion fluxassays and fluorescence assays using voltage-sensitive dyes (see, e.g.,Vestergarrd-Bogind et al., J. Membrane Biol., 88:67-75 (1988); Gonzales& Tsien, Chem. Biol., 4:269277 (1997); Daniel et al., J. Pharmacol.Meth., 25:185-193 (1991); Holevinsky et al., J. Membrane Biology,137:59-70 (1994)).

The effects of the test compounds upon the function of the polypeptidescan be measured by examining any of the parameters described above. Anysuitable physiological change that affects GPCR activity can be used toassess the influence of a test compound on the polypeptides of thisinvention. When the functional consequences are determined using intactcells or animals, one can also measure a variety of effects such astransmitter release, hormone release, transcriptional changes to bothknown and uncharacterized genetic markers (e.g., northern blots),changes in cell metabolism such as cell growth or pH changes, andchanges in intracellular second messengers such as Ca²⁺, IP3, cGMP, orcAMP.

Preferred assays for GPCRs include cells that are loaded with ion orvoltage sensitive dyes to report receptor activity. Assays fordetermining activity of such receptors can also use known agonists andantagonists for other G protein-coupled receptors as controls to assessactivity of tested compounds. In assays for identifying modulatorycompounds (e.g., agonists, antagonists), changes in the level of ions inthe cytoplasm or membrane voltage will be monitored using an ionsensitive or membrane voltage fluorescent indicator, respectively. Amongthe ion-sensitive indicators and voltage probes that may be employed arethose disclosed in the Molecular Probes 1997 Catalog. For Gprotein-coupled receptors, promiscuous G proteins such as Gα15 and Gα16can be used in the assay of choice (Wilkie et al., Proc. Nat'l Acad.Sci., 88:10049-10053 (1991)).

Receptor activation initiates subsequent intracellular events, e.g.,increases in second messengers. Activation of some G protein-coupledreceptors stimulates the formation of inositol triphosphate (IP3)through phospholipase C-mediated hydrolysis of phosphatidylinositol(Berridge & Irvine, Nature, 312:315-21 (1984)). IP3 in turn stimulatesthe release of intracellular calcium ion stores. Thus, a change incytoplasmic calcium ion levels, or a change in second messenger levelssuch as IP3 can be used to assess G protein-coupled receptor function.Cells expressing such G protein-coupled receptors may exhibit increasedcytoplasmic calcium levels as a result of contribution from both calciumrelease from intracellular stores and extracellular calcium entry viaplasma membrane ion channels.

In a preferred embodiment, T1R polypeptide activity is measured bystably or transiently co-expressing T1R genes, preferably stably, in aheterologous cell with a promiscuous G protein that links the receptorto a phospholipase C signal transduction pathway (see Offermanns &Simon, J. Biol. Chem., 270:15175-15180 (1995)). In a preferredembodiment, the cell line is HEK-293 (which does not normally expressT1R genes) and the promiscuous G protein is Gα15 (Offermanns & Simon,supra). Modulation of taste transduction is assayed by measuring changesin intracellular Ca²⁺ levels, which change in response to modulation ofthe T1R signal transduction pathway via administration of a moleculethat associates with T1R polypeptides. Changes in Ca²⁺ levels areoptionally measured using fluorescent Ca²⁺ indicator dyes andfluorometric imaging.

In another embodiment, phosphatidyl inositol (PI) hydrolysis can beanalyzed according to U.S. Pat. No. 5,436,128, herein incorporated byreference. Briefly, the assay involves labeling of cells with3H-myoinositol for 48 or more hrs. The labeled cells are treated with atest compound for one hour. The treated cells are lysed and extracted inchloroform-methanol-water after which the inositol phosphates wereseparated by ion exchange chromatography and quantified by scintillationcounting. Fold stimulation is determined by calculating the ratio of cpmin the presence of agonist, to cpm in the presence of buffer control.Likewise, fold inhibition is determined by calculating the ratio of cpmin the presence of antagonist, to cpm in the presence of buffer control(which may or may not contain an agonist).

Other receptor assays can involve determining the level of intracellularcyclic nucleotides, e.g., cAMP or cGMP. In cases where activation of thereceptor results in a decrease in cyclic nucleotide levels, it may bepreferable to expose the cells to agents that increase intracellularcyclic nucleotide levels, e.g., forskolin, prior to adding areceptor-activating compound to the cells in the assay. In oneembodiment, the changes in intracellular cAMP or cGMP can be measuredusing immunoassays. The method described in Offermanns & Simon, J. Bio.Chem., 270:15175-15180 (1995), may be used to determine the level ofcAMP. Also, the method described in Felley-Bosco et al., Am. J. Resp.Cell and Mol. Biol., 11: 159-164 (1994), may be used to determine thelevel of cGMP. Further, an assay kit for measuring cAMP and/or cGMP isdescribed in U.S. Pat. No. 4,115,538, herein incorporated by reference.

In another embodiment, transcription levels can be measured to assessthe effects of a test compound on signal transduction. A host cellcontaining T1R polypeptides of interest is contacted with a testcompound for a sufficient time to effect any interactions, and then thelevel of gene expression is measured. The amount of time to effect suchinteractions may be empirically determined, such as by running a timecourse and measuring the level of transcription as a function of time.The amount of transcription may be measured by using any method known tothose of skill in the art to be suitable. For example, mRNA expressionof the protein of interest may be detected using northern blots or theirpolypeptide products may be identified using immunoassays.Alternatively, transcription based assays using reporter gene may beused as described in U.S. Pat. No. 5,436,128, herein incorporated byreference. The reporter genes can be, e.g., chloramiphenicolacetyltransferase, luciferase, beta-galactosidase beta-lactamase andalkaline phosphatase. Furthermore, the protein of interest can be usedas an indirect reporter via attachment to a second reporter such asgreen fluorescent protein (see, e.g., Mistili & Spector, NatureBiotechnology, 15:961-964 (1997)).

The amount of transcription is then compared to the amount oftranscription in either the same cell in the absence of the testcompound, or it may be compared with the amount of transcription in asubstantially identical cell that lacks the T1R polypeptide(s) ofinterest. A substantially identical cell may be derived from the samecells from which the recombinant cell was prepared but which had notbeen modified by introduction of heterologous DNA. Any difference in theamount of transcription indicates that the test compound has in somemanner altered the activity of the T1R polypeptides of interest.

4. Transgenic Non-Human Animals Expressing Chemosensory Receptors

Non-human animals expressing a combination of T1R taste receptorsequences of the invention can also be used for receptor assays. Suchexpression can be used to determine whether a test compound specificallybinds to a mammalian taste transmembrane receptor complex in vivo bycontacting a non-human animal stably or transiently transfected withnucleic acids encoding chemosensory receptors or ligand-binding regionsthereof with a test compound and determining whether the animal reactsto the test compound by specifically binding to the receptor polypeptidecomplex.

Animals transfected or infected with the vectors of the invention areparticularly useful for assays to identify and characterize tastestimuli that can bind to a specific or sets of receptors. Suchvector-infected animals expressing human taste receptor sequences can beused for in vivo screening of taste stimuli and their effect on, e.g.,cell physiology (e.g., on taste neurons), on the CNS, or behavior.Alternatively, stable cell lines that express a T1R or combinationthereof, can be used as nucleic transfer donors to produced clonedtransgenic animals that stably express a particular T1R or combination.Methods of using nucleic transfer to produce cloned animals that expressa desired heterologous DNA are the subject of several issued U.S.patents granted to the University of Massachusetts (licensed to AdvancedCell Technology, Inc.) and Roslin Institute (licensed to Geron Corp.).

Means to infect/express the nucleic acids and vectors, eitherindividually or as libraries, are well known in the art. A variety ofindividual cell, organ, or whole animal parameters can be measured by avariety of means. The T1R sequences of the invention can be for exampleco-expressed in animal taste tissues by delivery with an infectingagent, e.g., adenovirus expression vector.

The endogenous taste receptor genes can remain functional and wild-type(native) activity can still be present. In other situations, where it isdesirable that all taste receptor activity is by the introducedexogenous hybrid receptor, use of a knockout line is preferred. Methodsfor the constitutive of non-human transgenic animals, particularlytransgenic mice, and the selection and preparation of recombinantconstructs for generating transformed cells are well known in the art.

Constitutive of a “knockout” cell and animal is based on the premisethat the level of expression of a particular gene in a mammalian cellcan be decreased or completely abrogated by introducing into the genomea new DNA sequence that serves to interrupt some portion of the DNAsequence of the gene to be suppressed. Also, “gene trap insertion” canbe used to disrupt a host gene, and mouse embryonic stem (ES) cells canbe used to produce knockout transgenic animals (see, e.g., Holzschu,Transgenic Res 6:97-106 (1997)). The insertion of the exogenous istypically by homologous recombination between complementary nucleic acidsequences. The exogenous sequence is some portion of the target gene tobe modified, such as exonic, intronic or transcriptional regulatorysequences, or any genomic sequence which is able to affect the level ofthe target gene's expression; or a combination thereof. Gene targetingvia homologous recombination in pluripotential embryonic stem cellsallows one to modify precisely the genomic sequence of interest. Anytechnique can be used to create, screen for, propagate, a knockoutanimal, e.g., see Bijvoet, Hum. Mol. Genet. 7:53-62 (1998); Moreadith,J. Mol. Med. 75:208-216 (1997); Tojo, Cytotechnology 19:161-165 (1995);Mudgett, Methods Mol. Biol. 48:167-184 (1995); Longo, Transgenic Res.6:321-328 (1997); U.S. Pat. Nos. 5,616,491; 5,464,764; 5,631,153;5,487,992; 5,627,059; 5,272,071; WO 91/09955; WO93/09222; WO 96/29411;WO 95/31560; WO 91/12650.

The nucleic acids of the invention can also be used as reagents toproduce “knockout” human cells and their progeny. Likewise, the nucleicacids of the invention can also be used as reagents to produce“knock-ins” in mice. The human or rat T1R gene sequences can replace theorthologous T1R in the mouse genome. In this way, a mouse expressing ahuman or rat T1R is produced. This mouse can then be used to analyze thefunction of human or rat T1Rs, and to identify ligands for such T1Rs.

a. Modulators

The compounds tested as modulators of a T1R family member can be anysmall chemical compound, or a biological entity, such as a protein,nucleic acid or lipid. Examples thereof include 5¹ IMP and 5¹ GMP.Essentially any chemical compound can be used as a potential modulatoror ligand in the assays of the invention, although most often compoundsthat are soluble in aqueous solutions are tested. Assays can be designedto screen large chemical libraries by automating the assay steps andproviding compounds from any convenient source; these assays aretypically run in parallel (e.g., in microtiter formats on microtiterplates in robotic assays). It will be appreciated that chemicallibraries can be synthesized by one of many chemical reactions (e.g.Senomyx proprietary chemistries). Additionally, there are many suppliersof chemical compounds, including Sigma (St. Louis, Mo.), Aldrich (St.Louis, Mo.), Sigma-Aldrich (St. Louis, Mo.), Fluka Chemika-BiochemicaAnalytika (Buchs, Switzerland) and the like.

In one preferred embodiment, high throughput screening methods involveproviding a combinatorial chemical or peptide library containing a largenumber of potential taste affecting compounds (potential modulator orligand compounds). Such “combinatorial chemical libraries” or “ligandlibraries” are then screened in one or more assays, as described herein,to identify those library members (particular chemical species orsubclasses) that display a desired characteristic activity. Thecompounds thus identified can serve as conventional “lead compounds” orcan themselves be used as potential or actual taste modulators.

Preferably, such libraries will be screened against cells or cell linesthat stably express a T1R or combination of T1Rs, i.e. T1R1/T1R3 orT1R2/T1R3 and preferably a suitable G protein, e.g. G_(α15). As shown inthe examples infra, such stable cell lines exhibit very pronouncedresponses to taste stimuli, e.g. umami or sweet taste stimuli. However,cells and cell lines that transiently express one or more T1Rs may alsobe used in such assays.

A combinatorial chemical library is a collection of diverse chemicalcompounds generated by either chemical synthesis or biologicalsynthesis, by combining a number of chemical “building blocks” such asreagents. For example, a linear combinatorial chemical library such as apolypeptide library is formed by combining a set of chemical buildingblocks (amino acids) in every possible way for a given compound length(i.e., the number of amino acids in a polypeptide compound). Thousandsto millions of chemical compounds can be synthesized through suchcombinatorial mixing of chemical building blocks.

Preparation and screening of combinatorial chemical libraries is wellknown to those of skill in the art. Such combinatorial chemicallibraries include, but are not limited to, peptide libraries (see, e.g.,U.S. Pat. No. 5,010,175, Furka, Int. J. Pept. Prot. Res., 37:487-493(1991) and Houghton et al., Nature, 354:84-88 (1991)). Other chemistriesfor generating chemically diverse libraries can also be used. Suchchemistries include, but are not limited to: peptoids (e.g., PCTPublication No. WO 91/19735), encoded peptides (e.g., PCT Publication WO93/20242), random bio-oligomers (e.g., PCT Publication No. WO 92/00091),benzodiazepines (e.g., U.S. Pat. No. 5,288,514), diversomers such ashydantoins, benzodiazepines and dipeptides (Hobbs et al., Proc. Nat.Acad. Sci., 90:6909-6913 (1993)), vinylogous polypeptides (Hagihara etal., J. Amer. Chem. Soc., 114:6568 (1992)), nonpeptidal peptidomimeticswith glucose scaffolding (Hirschmann et al., J. Amer. Chem. Soc.,114:9217-9218 (1992)), analogous organic syntheses of small compoundlibraries (Chen et al., J. Amer. Chem. Soc., 116:2661 (1994)),oligocarbamates (Cho et al., Science, 261:1303 (1993)), peptidylphosphonates (Campbell et al., J. Org. Chem., 59:658 (1994)), nucleicacid libraries (Ausubel, Berger and Sambrook, all supra), peptidenucleic acid libraries (U.S. Pat. No. 5,539,083), antibody libraries(Vaughn et al., Nature Biotechnology, 14(3):309-314 (1996) andPCT/US96/10287), carbohydrate libraries (Liang et al., Science,274:1520-1522 (1996) and U.S. Pat. No. 5,593,853), small organicmolecule libraries (benzodiazepines, Baum, C&EN, January 18, page 33(1993); thiazolidinones and metathiazanones, U.S. Pat. No. 5,549,974;pyrrolidines, U.S. Pat. Nos. 5,525,735 and 5,519,134; morpholinocompounds, U.S. Pat. No. 5,506,337; benzodiazepines, U.S. Pat. No.5,288,514, and the like).

Devices for the preparation of combinatorial libraries are commerciallyavailable (see, e.g., 357 UPS, 390 MHPS (Advanced Chem Tech, LouisvilleKy.), Symphony (Rainin, Woburn, Mass.), 433A (Applied Biosystems, FosterCity, Calif.), 9050 Plus (Millipore, Bedford, Mass.)). In addition,numerous combinatorial libraries are themselves commercially available(see, e.g., ComGenex, Princeton, N.J.; Tripos, Inc., St. Louis, Mo.; 3DPharmaceuticals, Exton, Pa.; Martek Biosciences; Columbia, Md.; etc.).

In one aspect of the invention, the T1R modulators can be used in anyfood product, confectionery, pharmaceutical composition, or ingredientthereof to thereby modulate the taste of the product, composition, oringredient in a desired manner. For instance, T1R modulators thatenhance sweet taste sensation can be added to sweeten a product orcomposition; T1R modulators that enhance umami taste sensation can beadded to foods to increase savory tastes. Alternatively, T1R antagonistscan be used to block sweet and/or umami taste.

b. Kits

T1R genes and their homologs are useful tools for identifyingchemosensory receptor cells, for forensics and paternity determinations,and for examining taste transduction. T1R family member-specificreagents that specifically hybridize to T1R nucleic acids, such as T1Rprobes and primers, and T1R specific reagents that specifically bind toa T1R polypeptide, e.g., T1R antibodies are used to examine taste cellexpression and taste transduction regulation.

Nucleic acid assays for the presence of DNA and RNA for a T1R familymember in a sample include numerous techniques are known to thoseskilled in the art, such as southern analysis, northern analysis, dotblots, RNase protection, S1 analysis, amplification techniques such asPCR, and in situ hybridization. In in situ hybridization, for example,the target nucleic acid is liberated from its cellular surroundings insuch as to be available for hybridization within the cell whilepreserving the cellular morphology for subsequent interpretation andanalysis. The following articles provide an overview of the art of insitu hybridization: Singer et al., Biotechniques, 4:230250 (1986); Haaseet al., Methods in Virology, vol. VII, pp. 189-226 (1984); and NucleicAcid Hybridization: A Practical Approach (Names et al., eds. 1987). Inaddition, a T1R polypeptide can be detected with the various immunoassaytechniques described above. The test sample is typically compared toboth a positive-control (e.g., a sample expressing a recombinant T1Rpolypeptide) and a negative control.

The present invention also provides for kits for screening formodulators of T1R family members. Such kits can be prepared from readilyavailable materials and reagents. For example, such kits can compriseany one or more of the following materials: T1R nucleic acids orproteins, reaction tubes, and instructions for testing T1R activity.Optionally, the kit contains a biologically active T1R receptor or cellline that stably or transiently expresses a biologically active T1Rcontaining taste receptor. A wide variety of kits and components can beprepared according to the present invention, depending upon the intendeduser of the kit and the particular needs of the user.

EXAMPLES

While the invention has been described in detail supra, the followingexamples are provided to illustrate preferred embodiments. Theseexamples are intended to be illustrative and not limitative of the scopeof the invention.

In the protein sequences presented herein, the one-letter code X or Xaarefers to any of the twenty common amino acid residues. In the DNAsequences presented herein, the one letter codes N or n refers to any ofthe of the four common nucleotide bases, A, T, C, or G.

Example 1 Production of Intronless hT1R Expression Constructs

Intronless hT1R expression constructs were cloned by a combination ofcDNA-based and genomic DNA-based methods. To generate the full-lengthhT1R1 expression construct, two 5′ coding exons identified in a clonedhT1R1 interval (accession #AL159177) were combined by PCR-overlap, andthen joined to a 5′-truncated testis cDNA clone. The hT1R2 expressionconstruct was generated from a partially sequenced hT1R2 genomicinterval. Two missing hT1R2 5′ exons were identified by screeningshotgun libraries of the cloned genomic interval using probes derivedfrom the corresponding rat coding sequence. Coding exons were thencombined by PCR-overlap to produce the full-length expression construct.The hT1R3 expression construct was generated by PCR-overlap from asequenced hT1R3 genomic interval (accession #AL139287). Rat T1R3 wasisolated from a rat taste tissue-derived cDNA library using an rT1R3exon fragment generated by hT1R3-based degenerate PCR. The partial hT1R1cDNA, rT1R2 cDNA, and partial hT1R2 genomic sequences were obtained fromDr. Charles Zuker (University of California, San Diego).

The nucleic acid and amino acid sequences for the above-identified T1Rcloned sequences as well as other full-length and partial T1R sequencesare set forth in the sequence listing.

Also, the following conceptual translations, which correspond to theC-termini of two orthologous pairs of fish T1Rs, are derived fromunpublished genomic sequence fragments and provided. Fugu T1RA wasderived from accession ‘scaffold 164’; Fugu T1RB was derived fromaccession LPC61711; Tetradon T1RA was derived from accession AL226735;Tetradon T1RB was derived from accession AL222381. Ambiguities in theconceptual translations (‘X’) result from ambiguities in databasesequences. These sequences can be found in the sequence listing.

Additionally, the accession number and reference citations relating tomouse and rat T1Rs and allelic variants thereof in the public domain areis set forth below: rT1R1 (Accession #AAD18069) (Hoon et al., Cell 96(4): 541-51 (1999)); rT1R2 (Accession #AAD18070) (Hoon et al., Cell96(4): 541-59 (1999)); mT1R1 (Accession #AAK39437); mT1R2 (Accession#AAK 39438); mT1R3 (Accession AAK 55537) (Max et al., Nat. Genet. 28(1):58-63 (2001)); rT1R1 (Accession #AAK7092) (Li et al., Mamm. Genome(12(1): 13-16 (2001)); mT1R1 (Accession #NP114073); mT1R1 (Accession#AAK07091) (Li et al., Mamm. Genome (121):13-16 (2001)); rT1R21(Accession #AAD18070) (Hoon et al., Cell 9664): 541-551 (1999)); mT1R2(Accession #NP114079); mT1R3 (Accession #AAK39436); mT1R3 (Accession#BAB47181); (Kitagawa et al., Biochem. Biophys. Res. Comm. 283(1):236-42(2001)); mT1R3 (Accession #NP114078); mT1R3 (Accession #AAK55536) (Maxet al., Nat. Genet. 28(1):58-63 (2001)); and mT1R3 (Accession No.AAK01937).

Example 2 Sequence Alignment of Human and Rat T1Rs

Cloned T1R sequences selected from those identified above were alignedagainst the corresponding rat T1Rs. As shown in FIG. 1, human T1R1,human T1R2 and human T1R3 and rat T1R3 were aligned with previouslydescribed T1Rs (rT1R1 having Accession #AAD18069 and rT1R2 havingAccession #AAD18070), the rat mGluR1 metabotropic, glutamate receptor(Accession #P23385); and the human calcium-sensing receptor (Accession#P41180). For clarity of the comparison, the mGluR1 and calcium-sensingreceptor C-termini are truncated. The seven potential transmembranesegments are boxed in blue. Residues that contact the glutamateside-chain carbutylate in the mGluR1 crystal structure are boxed in red,and residues that contact the glutamate α-amino acid moiety are boxed ingreen. The mGluR1 and calcium-sensing receptor cysteine residuesimplicated in intersubunit disulfide-based formation are circled inpurple. These cysteines are not conserved in T1R1 and T1R2, but arelocated in a degraded region: of the alignment that contains apotentially analogous T1R3 cysteine residue, also circled.

Example 3 Demonstration by RT-PCR that hT1R2 and hT1R3 are Expressed inTaste Tissue

As shown in FIG. 2, hT1R2 and hT1R3 are expressed in taste tissue:expression of both genes can be detected by RT-PCR from resected humancircumvallate papillae.

Example 4 Methods for Heterologous Expression of T1Rs in HeterologousCells

An HEK-293 derivative (Chandrashekar et al., Cell 100(6): 703-11(2000)), which stably expresses Gα15, was grown and maintained at 37° C.in Dulbecco's Modified Eagle Medium (DMEM, Gibco BRL) supplemented with10% FBS, MEM non-essential amino acids (Gibco BRL), and 3 μg/mlblasticidin. For calcium-imaging experiments, cells were first seededonto 24-well tissue-culture plates (approximately 0.1 million cells perwell), and transfected by lipofection with Mirus TransIt-293 (PanVera).To minimize glutamate-induced and glucose-induced desensitization,supplemented DMEM was replaced with low-glucose DMEM/GlutaMAX (GibcoBRL) approximately 24 hours after transfection. 24 hours later, cellswere loaded with the calcium dye Fluo-4 (Molecular Probes), 3 μM inDulbecco's PBS buffer (DPBS, GibcoBRL), for 1.5 hours at roomtemperature. After replacement with 250 μl DPBS, stimulation wasperformed at room temperature by addition of 200 μl DPBS supplementedwith taste stimuli. Calcium mobilization was monitored on a AxiovertS100 TV microscope (Zeiss) using Imaging Workbench 4.0 software (Axon).T1R1/T1R3 and T1R2/T1R3 responses were strikingly transient—calciumincreases rarely persisted longer than 15 seconds—and asynchronous. Thenumber of responding cells was thus relatively constant over time;therefore, cell responses were quantitated by manually counting thenumber of responding cells at a fixed time point, typically 30 secondsafter stimulus addition.

Example 5 Human T1R2/T1R3 Functions as a Sweet Taste Receptor

HEK cells stably expressing Gα15 were transiently transfected with humanT1R2, T1R3 and T1R2/T1R3, and assayed for increases in intracellularcalcium in response to increasing concentrations of sucrose (FIG. 3(a)). Also, T1R2/T1R3 dose responses were determined for several sweettaste stimuli (FIG. 3( b)). The maximal percentage of responding cellswas different for different sweeteners, ranging from 10-30%. Forclarity, dose responses were normalized to the maximal percentage ofresponding cells. The values in FIG. 3 represent the mean ±s.e. of fourindependent responses. X-axis circles mark psychophysical detectionthresholds determined by taste testing. Gurmarin (50-fold dilution of afiltered 10 g/l Gymnema sylvestre aqueous extract) inhibited theresponse of T1R2/T1R3 to 250 mM sucrose, but not the response ofendogenous β2-adrenergic receptor to 20 μM isoproterenol (FIG. 3( b)).FIG. 3( c) contains the normalized response of T1R2/T1R3 co-expressingcell lines to different sweeteners (sucrose, aspartame, D-tryptophan andsaccharin).

Example 6 Rat T1R2/T1R3 also Functions as a Sweet Taste Receptor

HEK cells stably expressing Gα15 were transiently transfected withhT1R2/hT1R3, rT1R2/rT1R3, hT1R2/rT1R3, and rT1R2/hT1R3. Thesetransfected cells were then assayed for increased intracellular calciumin response to 350 mM sucrose, 25 mM tryptophan, 15 mM aspartame, and0.05% of monellin. The results with sucrose and aspartame are containedin FIG. 4 and indicate that rT1R2/rT1R3 also functions as a sweet tastereceptor. Also, these results suggest that T1R2 may control T1R2/T1R3ligand specificity.

Example 7 T1R2/T1R3 Responses Using an Automated Fluorescence BasedAssay

HEK cells stably expressing Gα15 were transiently transfected with hT1R2and hT1R3. These cells were loaded with the calcium dye Fluo-4, andtheir responses to a sweetener measured using a fluorescence platereader. FIG. 5 contains cyclamate (12.5 mM) responses for cellsexpressing hT1R2/hT1R3 and for cells expressing only hT1R3 (J19-22). Thefluorescence results obtained indicate that responses to these tastestimuli only occurred in the cells expressing hT1R2/hT1R3. FIG. 6contains normalized dose-response curves, the results of which show thathT1R2 and hT1R3 function together as a human taste receptor based ontheir dose-specific interaction with various sweet stimuli.Particularly, FIG. 6 contains dose-responses for sucrose, tryptophan andvarious other commercially available sweeteners. These results indicatethat T1R2/T1R3 is a human sweet taste receptor as the rank order andthreshold values obtained in the assay closely mirror values for humansweet taste.

Example 8 Ligand-Binding Residues of mGluR1 are Conserved in T1R1

As shown in FIG. 6, the key ligand-binding residues of mGluR1 areconserved in T1R1. The interaction of glutamate with mGluR1 is shownwith several key residues highlighted according to the same color schemeas FIG. 1.

Example 9 Human T1R1/T1R3 Functions as Umami Taste Receptors

HEK cells stably expressing Gal 5 were transiently transfected withhuman T1R1, T1R3 and T1R1/T1R3 and assayed for increases inintracellular calcium in response to increasing concentrations ofglutamate (FIG. 8( a)), and 0.5 mM glutamate), 0.2 mM IMP, and 0.5 mMglutamate plus 0.2 mM IMP (FIG. 8( b)). Human T1R1/T1R3 dose responseswere determined for glutamate in the presence and absence of 0.2 mM IMP(FIG. 8( c)). The maximal percentages of responding cells wasapproximately 5% for glutamate and approximately 10% for glutamate plusIMP. For clarity, does responses are normalized to the maximalpercentage of responding cells. The values represent the mean ±s.e. offour independent responses. X-axis circles mark taste detectionthresholds determined by taste testing.

Example 10 PDZIP as an Export Sequence

The six residue PDZIP sequence (SVSTW (SEQ ID NO:1)) was fused to theC-terminus of hT1R2 and the chimeric receptor (i.e. hT1R2-PDZIP) wastransfected into an HEK-293 host cell. The surface expression of hT1R2was then monitored using immunofluorescence and FACS scanning data. Asshown in FIGS. 9A and 9B, the inclusion of the PDZIP sequence increasedthe surface expression of hT1R2-PDZIP relative to hT1R2. Morespecifically, FIG. 9A shows an immunofluorescence staining of myc-taggedhT1R2 demonstrating that PDZIP significantly increases the amount ofhT1R2 protein on the plasma membrane. FIG. 9B shows FACS analysis datademonstrating the same result—Cells expressing myc-tagged hT1R2 areindicated by the dotted line and cells expressing myc-tagged-hT1R2-PDZIPare indicated by the solid line. Particularly, FIG. 10A showsuntransfected Gα15 stable host cells in HBS buffer, FIG. 10B showshT1R2-PDZIP transfected Gα15 stable hose cells in sweetener pool no. 5(saccharin, sodium cyclamate, Acesulfame K, and Aspartame-20 mM each inHBS buffer), FIG. 10C shows T1R3-PDZIP transfected Gα15 stable hostcells in sweetener pool no. 5, and FIG. 10D showshT1R2-PDZIP/hT1R3-PDZIP co-transfected Gα15 stable host cells insweetener pool no. 5. Further, FIGS. 10E-10H show dose-dependentresponse of hT1R2/hT1R3 co-transfected Gα15 stable host cells tosucrose-E: 0 mM in HBS buffer; F: 30 mM; G: 60 mM; and H: 250 mM. FIGS.10I-10L shown the responses of hT1R2/hT1R3 co-transfected Gα15 stablehost cells to individual sweeteners—I: Aspartame (1.5 mM); J: AcesulfaneK (1 mM); K: Neotame (20 mM); L: Sodium cyclamate (20 mM). Asdemonstrated by the calcium-images of FIG. 10, hT1R2 and hT1R3 are bothrequired for the activities triggered by the sweet stimuli.

Example 11 Generation of Cell Lines that Stably Co-Express T1R1/T1R3 orT1R2/T1R3

Human cell lines that stably co-express human T1R2/T1R3 or humanT1R1/T1R3 were generated by transfecting linearized PEAK10-derived (EdgeBiosystems) vectors and pcDNA 3.1/ZEO-derived (Invitrogen) vectorsrespectively containing hT1R1 or hT1R2 expression construct (plasmidSAV2485 for T1R1, SAV2486 for T1R2) and hT1R3 (plasmid SXV550 for T1R3)into a G_(α15) expressing cell line. Specifically, T1R2/T1R3 stable celllines were produced by co-transfecting linearized SAV2486 and SXV550into Aurora Bioscience's HEK-293 cell line that stably expressesG_(α15). T1R1/T1R3 stable cell lines were produced by co-transfectinglinearized SAV2485 and SXV550 into the same HEK-293 cell line thatstably expresses G_(α15). Following SAV2485/SXV550 and SAV2486/SXV550transfections, puromycin-resistant and zeocin-resistant colonies wereselected, expanded, and tested by calcium imaging for responses to sweetor umami taste stimuli. Cells were selected in 0.0005 mg/ml puromycin(CALBIOCHEM) and 0.1 mg/ml zeocin (Invitrogen) at 37° C. in low-glucoseDMEM supplemented with GlutaMAX, 10% dialyzed FBS, and 0.003 mg/mlblasticidin. Resistant colonies were expanded, and their responses tosweet taste stimuli evaluated by Fluorescence microscopy. For automatedfluorimetric imaging on VIPR-II instrumentation (Aurora Biosciences),T1R2/T1R3 stable cells were first seeded onto 96-well plates(approximately 100,000 cells per well). Twenty-four hours later, cellswere loaded with the calcium dye fluo-3-AM (Molecular Probes), 0.005 mMin PBS, for one hour at room temperature. After replacement with 70 μlPBS, stimulation was performed at room temperature by addition of 70 μlPBS supplemented with taste stimuli. Fluorescence (480 nm excitation and535 nm emission) responses from 20 to 30 seconds following compoundaddition were averaged, corrected for background fluorescence measuredprior to compound addition, and normalized to the response to 0.001 mMionomycin (CALBIOCHEM), a calcium ionophore.

It was then observed that when these cell lines were exposed to sweet orumami stimuli, that for active clones typically 80-100% of cellsresponded to taste stimuli. Unexpectedly, the magnitude of individualcell responses was markedly larger than that of transiently transfectedcells.

Based on this observation, the inventors tested the activity of T1Rstable cell lines by automated fluorescence imaging using AuroraBioscience's VIPR instrumentation as described above. The responses oftwo T1R1/T1R3 and one T1R2/T1R3 cell line are shown in FIG. 11 and FIG.12 respectively.

Remarkably, the combination of increased numbers of responding cells andincreased response magnitudes resulted in a greater than 10-foldincrease in activity relative to transiently transfected cells. (By wayof comparison, the percent ionomycin response for cells transientlytransfected with T1R2/T1R3 was approximately 5% under optimalconditions.) Moreover, dose responses obtained for stably expressedhuman T1R2/T1R3 and T1R1/T1R3 correlated with human taste detectionthresholds. The robust T1R activity of these stable cell lines suggeststhat they are well suited for use in high-throughput screening ofchemical libraries in order to identify compounds, e.g. small molecules,that modulate the sweet or umami taste receptor and which thereforemodulate, enhance, block or mimic sweet or umami taste.

Example 12 Generation of Cell Lines that Inducibly Co-Express T1R1/T1R3which Selectively Respond to Umami Taste Stimuli

T1R1/T1R3 HEK 293 cell lines that stably expressed the umami tastereceptor display robust improved activity relative to transientlytransfected cites. However, a disadvantage is that they can rapidly loseactivity during cell propagation.

Also, these findings show that (i) T1R1/T1R3 is a umami taste receptor,i.e., and (ii) that cell lines which robustly express T1R1/T1R3,preferably stable and/or inducible T1R1/T1R3 cell lines can be used inassays, preferably for high throughput screening of chemical librariesto identify novel modulators of umami taste. Modulators that enhanceumami taste may be used.

To overcome the instability of the T1R1/T1R3 stable cell lines, theHEK-G_(α15) cells have been engineered to inducibly express T1R1/T1R3using the GeneSwitch system (Invitrogen). pGene-derived zeocin-resistantexpression vectors for human T1R1 and T1R3 (plasmid SXV603 for T1R1 andSXV611 for T1R3) and a puromycin-resistant pSwitch-derived vector thatcarries the GeneSwitch protein (plasmid SXV628) were linearized andcotransfected into the HEK-G_(α15) cell line. Zeocin-resistant andpuromycin-resistant colonies were selected, expanded, induced withvariable amounts of mifepristone, and tested by calcium imaging forresponses to umami taste stimuli.

Inducible expression of T1R1/T1R3 resulted in robust activity. Forexample, approximately 80% of induced cells but only approximately 10%of transiently transfected cells responded to L-glutamate; Morespecifically, pGene derived Zeocin-resistant expression vectors thatexpress human T1R1 and human T1R3 and a puromycn-resistantpSwitch-derived vector that carries the GeneSwitch protein werelinearized and co-transfected into G_(α15) cells. Cells were selected in0.5 μg/ml puromycin (CAL BIOCHEM) and 100 μg/ml Zeocin (Invitrogen) at37° C. in Dulbecco's Modified Eagle Medium supplemented with GlutaMAX,(10% dialyzed FBS, and 3 ug/ml blasticidin. Resistant colonies wereexpanded, and their responses to umami taste stimuli following inductionwith 10⁻¹⁰ M mifepristone determined by fluorescence microscopyfollowing the methods of Li et al., PNAS 99(7): 4692-4696 (2002).

For automated fluorometric imaging on FLIPR instrumentation (MolecularDevice), cells from one clone (designated clone I-17) were seeded into96-well plates (approximately 80,000 cell per well) in the presence of10⁻¹⁰ M mifepristone and incubated for 48 hours. Cells were then loadedwith the calcium dye fluo-4-AM (Molecular Probes), 3 μM in PBS, for 1.5hours at room temperature.

After replacement with 50 μl PBS, stimulation was performed at roomtemperature by the addition of 50 μl PBS supplemented with differentstimuli. In contrast to previous transient T1R1/T1R3 umami receptorexpression systems that necessitated quantifying T1R1/T1R3 receptoractivity by individually counting responding cells (Li et al., PNAS99(7): 4692-4696 (2002)) (because of the low activity of the receptortherein), the subject inducible expression system resulted in a cloneI-17 having substantially increased activity that allowed receptoractivity to be quantified by determining maximal fluorescence increases(480 nm excitation and 535 nm emission) summated over fields of imagedcells. The maximal fluorescence from four independent determinations wasaveraged, corrected for background fluorescence measured prior tocompound addition, and normalized to the response to 0.002 mM ionomycin(CALBIOCHEM).

These results are contained in FIG. 13. Particularly, FIG. 13 contains adose-response curve determined for L-glutamate in the presence andabsence of 0.2 mM IMP. In the figure, each value, represents averagesummated maximal fluorescence (corrected for background fluorescence)for four independent determinations. These dose-response curvescorrespond to those determined for cells transiently transfected withT1R1/T1R3.

The selectivity of the umami T1R1/T1R3 taste receptor was also evaluatedby screening with different L-amino acids. The results obtainedindicated that T1R1/T1R3 is selectively activated by the umami-tastingL-amino acids (L-glutamate and L-aspartate).

The results of experiments wherein the responses of the I-17 clone wasresulted in tested in the presence of different L-amino acids arecontained in FIG. 14 and FIG. 15. FIG. 14 shows the results of anexperiment wherein the I-17 cell line was contacted with differentL-amino acids at a concentration of 10 mM in the presence and absence of1 mM IMP.

FIG. 15 contains a dose-response curve for active amino acids determinedin the presence of 0.2 mM IMP. Each: value represents the average offour independent determinations.

The results obtained in these experiments support the specificity andselectivity of the umami taste receptor to umami taste stimuli. Whereasthe umami taste stimuli L-glutamate and L-aspartate significantlyactivated the T1R1/T1R3 receptor at different concentrations (see FIGS.14 and 15), the other L-amino acids which activated the human T1R1/T1R3receptor only activated the receptor weakly and at much higherconcentrations.

Therefore, these results support the selectivity of the T1R1/T1R3receptor for umami taste stimuli and the suitability of this induciblestable expression system for use in high throughput screening assaysusing automated fluorometric imaging instrumentation to identifycompounds that activate the umami taste receptor, for exampleL-glutamate or L-aspartate, or which enhance the activity of L-glutamateto activate the umami taste receptor, for example 5′-IMP or 5′-GMP, orblock the activation of the umami taste receptor by umami taste stimulisuch as L-glutamate and L-aspartate.

Compounds identified using these assays have potential application asflavorants in foods and beverage compositions for mimicking or blockingumami taste stimuli.

Example 13 Lactisole Inhibits the Receptor Activities of Human T1R2/T1R3and T1R1/T1R3, and Sweet and Umami Taste

Lactisole, an aralkyl carboxylic acid, was thought to be a selectivesweet-taste inhibitor (See e.g., Lindley (1986) U.S. Pat. No. 4,567,053;and Schiffman et al. Chem Senses 24:439-447 (1999)). Responses ofHEK-G_(α15) cells transiently transfected with T1R2/T1R3 to 150 mMsucrose in the presence of variable concentrations of lactisole weremeasured. Lactisole inhibits the activity of human T1R2/T1R3 with anIC₅₀ of 24 μM.

The T1R1/T1R3 umami and T1R2/T1R3 sweet taste receptor may share acommon subunit. It has therefore been theorized that lactisole, whichinhibit the T1R2/T1R3 sweet taste receptor, may have a similar effect onthe T1R1/T1R3 umami taste receptor. The present inventors tested theeffect of lactisole on the response of human T1R1/T1R3 to 10 mML-Glutamate. As with the T1R2/T1R3 sweet receptor, lactisole inhibitedT1R1/T1R3 with an IC₅₀ of 165 μM. Lactisole inhibition likely reflectsantagonism at the T1R receptors instead of, for example, non-specificinhibition of G_(α15)-mediated signaling because the response ofmuscarinic acetylcholine receptors was not inhibited by lactisole.

The present inventors then evaluated the effect of lactisole on humanumami taste. Taste thresholds in the presence of 1 and 2 mM lactisolewere determined for the umami taste stimuli L-Glutamate with or without0.2 mM IMP, the sweet taste stimuli sucrose and D-tryptophan, and thesalty taste stimulus sodium chloride following the methods of Schiffmanet al. (Chem. Senses 24: 439-447 (1989)). Millimolar concentrations oflactisole dramatically increased detection thresholds for sweet andumami but not salt taste stimuli. These results are contained in FIG.16.

In conclusion, (i) these findings further support the inventors'hypothesis that T1R/T1R3 is the only umami taste receptor, and (ii) theT1R1/T1R3 and T1R2/T1R3 receptors may share a structurally relatedlactisole-binding domain.

While the foregoing detailed description has described severalembodiments of the present invention, it is to be understood that theabove description is illustrative only and not limiting of the disclosedinvention. The invention is to be limited only by the claims whichfollow.

Example 14 Mapping of Ligand Interaction Sites on the Sweet Receptor

Through coexpression of T1R2R-H with human T1R3, part of the human sweetreceptor (the N-terminal domain of T1R2) was replaced with rat proteinsequence. The responses to aspartame and neotame are abolished, showingthat the N-terminal domain of human T1R2 is required for recognizingaspartame and neotame. Similarly, the rat T1R2 N-terminal domain wasalso replaced with human protein sequence by coexpressing T1R2H-R withrat T1R3. The chimeric receptor gains the ability to respond toaspartame and neotame, suggesting that the same domain of human T1R2 isalso sufficient (in the context of sweet receptors) to recognize thosetwo sweeteners (FIG. 22B). These in vitro functional expression dataindicate that the important interaction determinants are located in theN-terminal extracellular domain.

In contrast, replacing either half of human T1R2 with rat proteinsequence does not affect its response to cyclamate. Instead, theC-terminal domain of human T1R3 is required and sufficient, whenco-expressed with T1R2, to recognize cyclamate (FIG. 22C). Thetransmembrane domain of family C GPCRs has been known to contain bindingsites for allosteric modulators (Gasparini, F., R. Kuhn, and J. P. Pin,Curr Opin Pharmacol 2002 February; 2(1):43-9). This is the first case infamily C GPCR, where an agonist binds directly to the transmembranedomain and activates the receptor in the absence of other ligand.

Lactisole, an aralkyl carboxylic acid, is a specific human sweet tasteinhibitor, which has physiological effect on the rodent taste.Consistent with the taste effect, lactisole inhibits the human but notrat T1R2/T1R3 response to sucrose in our assay system (FIG. 22A). Thesame kind of mapping experiments on lactisol interaction site using theT1R chimeras was performed. Like cyclamate, lactisole requires the humanT1R3C-terminal domain to inhibit the receptor's response to sucrose andacesulfame K

(FIG. 22D). This result further demonstrates the importance ofT1R3C-terminal domain in the sweet receptor function. The chimeras inall 16 possible combinations were tested, and all functionalcombinations generated results consistent with our model.

Mutagenesis studies were conducted on both T1R2 and T1R3 to narrow downthe essential amino acids in recognition of aspartame, neotame, andcyclamate. If T1R2 and T1R3 are responsible for recognizing differentsweeteners, mutations in T1R2 N-terminal domain would affect responsesto aspartame and neotame, but not cyclamate. In addition, mutations inT1R3C-terminal domain would have the opposite effect. To select thecrucial amino acid residues in the T1R2 N-terminal domain, the sequenceof T1R2 was aligned with mGluR1 (FIG. 23A). Among the eight residuesthat are crucial in ligand binding in mGluR1 (Kunishima, N., et al.,Nature, 2000. 407(6807): p. 971-7), three are conserved in human T1R2(S144, Y218, and E302). Each of the three residues were mutated and theresulting receptors were tested for their response to differentsweeteners. Substitution of Y218 to A abolished the responses to allsweeteners tested, showing Y218 is important for the overallconformation of the receptor. The two other hT1R2 variants, containingS144A and E302A, selectively affected the response to aspartame andneotame but not cyclamate. Stable cell lines expressing S144A and E302AhT1R2 variants (coexpressed with wild type hT1R3 and G_(α15)) did notrespond to aspartame or neotame at the physiological concentrations, butdid respond to cyclamate (FIG. 23B).

In order to further map the cyclamate-binding site, the threeextracellular loops in the T1R3C-terminal domain were focused on.Alignment of human and rodent T1R3s reveal multiple amino aciddifferences in the three extracellular loops (FIG. 23C). Replacingextracellular loop-2 or loop-3 with rat sequences abolished thecyclamate response without affecting the sucrose or aspartame responses.In contrast, replacing extracellular loop 1 had no obvious effect onresponse to cyclamate, showing an important role for EC loops 2 and 3 inrecognizing cyclamate (FIG. 23D). None of those loop-replacementsaffected the inhibition effect of lactisole, showing a different bindingmechanism. In summary, amino acid substitutions in T1R2 or T1R3 resultin selective interference of activities induced by different sweeteners,consistent with the chimeric receptor results.

The above results demonstrate that the human sweet receptor function asa heteromeric complex of T1R2 and T1R3. Both subunits are required forrecognizing different sweeteners, and the data indicate the existence ofmultiple binding pockets on the receptor for different classes ofagonists. The presence of multiple ligand-binding sites providesstructural guidance and definition for the specifically bindingcompounds of the invention.

Example 15 Mapping of Receptor-G Protein Interactions

The human and rat sweet receptors are also different in their Gprotein-coupling efficiency. Even though both human and rat receptorscan couple efficiently to G_(α15/il), only the human receptor can coupleefficiently to G_(α15) (FIG. 24A). This species difference allows formapping of the receptor G protein interactions using the same chimericreceptors as described above. T1R2 but not T1R3 appears to be criticalfor Gals-coupling, since replacing the C-terminus of human T1R2 with thecorresponding rat sequence abolished coupling, and replacing ratT1R2C-terminal half with human sequence enabled the receptor to coupleto G_(α15) and respond to sucrose and acesulfame K (FIG. 24); Swappingthe T1R3C-terminal sequences had no effect on G_(α15)-coupling (FIG.24B). This observation demonstrates the important role of T1R2 in Gprotein-coupling in the functional expression system. Gustducin (Wong,G. T., K. S. Gannon, and R. F. Margolskee, Nature, 1996. 381(6585): p.796-800) has been proposed to be an endogenous G protein for the sweettaste receptor, and T1R2 can be the subunit responsible for in vivocoupling in taste cells. GABA_(B)R is the other example of heteromericfamily C GPCR, whereas one subunit (GABA_(B)R1) is responsible forligand-binding, and the other (GABA_(B)R2) for G protein coupling(Margeta-Mitrovic, M., Paroc Natl Acad Sci USA, 2001. 98(25): p.14643-8; Margeta-Mitrovic, M., Proc Natl Acad Sci USA, 2001. 98(25): p.14649-54). The sweet receptor is different from GABA_(B)R in that T1R2is required for both ligand recognition and G-protein coupling.

Example 16 Lactisole Antagonizes Human T1R1/T1R3 and Inhibits HumanUmami Taste

It was hypothesized that since T1R1/T1R3 function as heteromericreceptors as well as the sweet receptor, that lactisole should havesimilar effect on T1R1/T1R3 activity, since T1R3 is a common subunitbetween the sweet and the umami receptors. Indeed, lactisole antagonizedhuman T1R1/T1R3 (FIG. 25A). Lactisole acts as a noncompetitive inhibitorof T1R1/T1R3, since the IC₅₀ values are apparently not dependent onglutamate concentration, (FIG. 25B), and lactisole reduces the maximalactivities of the receptor without significantly changing the EC₅₀ ofagonists (FIG. 25C). These results demonstrate that lactisole binds to adifferent site from L-glutamate, and are consistent with the hypothesisthat the glutamate-binding pocket is located in T1R1. Lactisole appearsto be a competitive inhibitor of the sweet receptor, as its IC₅₀s aredependent on the concentrations of the sweeteners, and it increases theEC₅₀s of the sweeteners without significantly affecting the maximalactivities.

The inhibition effect of lactisole is mediated by the T1R receptorssince it had no effect on the endogenous muscarinic acetylcholinereceptor in HEK cells or on a mouse bitter receptor, mT2R5, transientlyexpressed in HEK cells. As was the case for the T1R2/T1R3 receptor,lactisole inhibition of the T1R1/T1R3 response to umami taste stimuliwas reversible following washout and restimulation.

To correlate the receptor activity with behavior, the effect oflactisole on human umami taste was tested. As predicted, millimolarconcentrations of lactisole dramatically increased detection thresholdsfor sweet and umami but not salt taste stimuli (FIG. 25D). Lactisole waspreviously not known to be an umami taste inhibitor. The correlationbetween receptor activity and taste results demonstrates a crucial roleof T1Rs in human umami taste.

Example 17 Cyclamate Enhances Human T1R1/T1R3 Receptor Activities

Based on the same heteromeric model of T1Rs (FIG. 26), it was predictedthat cyclamate would also modulate the activity of the human T1R1/T1R3umami receptor by acting on T1R3. Although cyclamate alone had no effecton T1R1/T1R3, it enhanced the activity of the receptor in the presenceof L-glutamate (FIG. 27E). This effect is specific for the humanT1R1/T1R3, as cyclamate had no effect on the activities of theendogenous muscarinic acetylcholine receptor in the presence ofcarbachol (FIG. 27E). It is noteworthy that cyclamate has comparableEC₅₀s for the sweet receptor (FIG. 23B) and umami receptor. Cyclamatereproducibly left-shifts the dose-response curves for L-glutamate by

˜2 fold in the presence or absence of IMP (FIG. 25F). IMP has a moredramatic effect of enhancing the receptor, and the effect of cyclamateis observed in the presence of IMP (FIG. 25F), suggesting a differentmechanism from IMP in enhancing the receptor. IMP appears to bind toT1R1, since it has no effect on the sweet receptor. Other sweeteners,including sucrose, aspartame, saccharin, and D-tryptophan, had no effecton the human T1R1/T1R3 activities.

In summary, it has been demonstrated that both T1R2 and T1R3 arerequired in a functional sweet receptor, that aspartame and neotamerequire the N-terminal extracellular domain of T1R2, G protein-couplingrequires C-terminal half of T1R2, and that cyclamate and lactisolerequire the transmembrane domain of T1R3. These findings demonstrate thedifferent functional roles of T1R subunits in a heteromeric complex andthe presence of multiple sweetener interaction sites on the sweetreceptor. Because T1R3 is the common subunit in the sweet and the umamireceptors, it was predicted and confirmed the effect of cyclamate andlactisole on the umami receptor. Furthermore, a correlation was able tobe made between the lactisole effect on the receptor activities withtaste. Based on these observations, a model was created (FIG. 26) forthe structure-function relationships of the T1R family taste receptors.Natural carbohydrate sweeteners bind to the N-terminal domain of T1R2,similar to aspartame and neotame, and there are other ligand bindingsites on the sweet receptor as well, for example, the transmembranedomain of T1R2. The umami receptor functions similarly as a heteromericcomplex, and MSG and IMP each appears to bind to the T1R1 subunit, sinceneither has any effect on the sweet receptor, and the transmembranedomain of T1R1 is responsible for coupling to G proteins.

Example 18 HTS Protocol for Sweet Tastants

An HEK293 cell line derivative (Chandrashekar, J., Mueller, K. L., Hoon,M. A., Adler, E., Feng, L., Guo, W., Zuker, C. S., Ryba, N. J., Cell,2000, 100, 703-711.) that stably expresses Gα15 and hT1R2/hT1R3 (Li, X.,Staszewski, L., Xu, H., Durick, K., Zoller, M., Adler, E. Proc Natl AcadSci USA 2002, 99, 4692-4696, World Patent #WO 03/001876 A2, hereinincorporated by reference in their entirety) was used to identifycompounds with sweet taste enhancing properties.

Compounds were initially selected based on their activity on thehT1R2/hT1R3-HEK293-Gα15 cell line (Li, et al. vide supra). Activity wasdetermined using an automated fluorometric imaging assay on a FLIPRinstrument (Fluorometric Intensity Plate Reader, Molecular Devices,Sunnyvale, Calif.) (designated FLIPR assay). Cells from one clone(designated S-9 cells) were seeded into 384-well plates (atapproximately 50,000 cells per well) in a medium containing DMEM LowGlucose (Invitrogen, Carlsbad, Calif.), 10% dialyzed fetal bovine serum(Invitrogen, Carlsbad, Calif.), 100 Units/ml Penicillin G, and 100 μg/mlStreptomycin (Invitrogen, Carlsbad, Calif.) (Li, et al. vide supra) seealso World Patent #WO 03/001876 A2). S-9 cells were grown for 24 hoursat 37° C. S-9 cells were then loaded with the calcium dye Fluo-3AM(Molecular Probes, Eugene, Oreg.), 4 μM in a phosphate buffered saline(D-PBS) (Invitrogen, Carlsbad, Calif.), for 1 hour at room temperature.After replacement with 25 μl D-PBS, stimulation was performed in theFLIPR instrument and at room temperature by the addition of 25 μl D-PBSsupplemented with different stimuli at concentrations corresponding totwice the desired final level. Receptor activity was quantified bydetermining the maximal fluorescence increases (using a 480 nmexcitation and 535 nm emission) after normalization to basalfluorescence intensity measured before stimulation.

For dose-responses analysis stimuli were presented in duplicates at 10different concentrations ranging from 60 nM to 30 μM. Activities werenormalized to the response obtained with 400 mM D-fructose, aconcentration that elicits maximum receptor response. EC₅₀s weredetermined using a non-linear regression algorithm (using Senomyx, Inc.software), where the Hill slope, bottom asymptotes and top asymptoteswere allow to vary. Identical results were obtained when analyzing thedose-response data using commercially available software for non-linearregression analysis such as GraphPad PRISM (San Diego, Calif.).

In order to determine the dependency of hT1R2/hT1R3 for the cellresponse to different stimuli, selected compounds were subjected to asimilar analysis on HEK293-Gα15 cells (not expressing the human sweetreceptor). The HEK293-Gα15 cells do not show any functional response inthe FLIPR assay to D-Fructose or any other known sweeteners. Similarly,compounds described herein do not induce any functional response whenusing HEK293-Gα15 cells in the FLIPR assay.

Example 19 Flavor Enhancement Measurements for Sweet Tastants UsingHuman Volunteers

Basic screening of sensory taste testers: Potential panelists weretested for their abilities to rank and rate intensities of solutionsrepresenting the five basic tastes. Panelists ranked and rated intensityof five different concentrations of each of the five followingcompounds: sucrose (sweet), sodium chloride (salty), citric acid (sour),caffeine (bitter), and monosodium glutamate (umami). Panelists tasted atotal of 25 samples per session (5 samples of each of the 5 solutiontypes). In the first session, panelists ranked the five concentrationsfor intensity of the attribute in question. This was repeated four moretimes with other samples. In the second session, panelists ratedintensity of the five concentrations of each sample using a line scalecalled the “Labeled Magnitude Scale” (LMS). The LMS is anchored withintensities (e.g. barely detectable, weak, moderate, strong, verystrong, and strongest imaginable) to assist panelists in rating thesamples. Samples were tasted in 10 ml volumes at room temperature andlabeled with 3-digit blinding codes. Samples were presented inrandomized, counterbalanced order within each sample solution (e.g.sucrose, citric acid, etc.).

In order to be selected for participation in testing, panelists neededto correctly rank and rate samples for intensity, with a reasonablenumber of errors. Approximately 25 people successfully completed thisprocedure.

Panelists selected in the above procedure were deemed qualified forperforming Preliminary Taste Testing procedures. The preliminary tastetests are used to evaluate new compounds for intensity of basic tastesand off-tastes. A small group of panelists (n=5) taste approximately 5concentrations of the compound (range typically between I-100 uM, inhalf-log cycles, e.g. 1, 3, 10, 30, and 100 uM) in water or buffer andin a solution of 4% (w/v, 117 mM) sucrose to evaluate enhancement.Typically samples also contain 0.1% ethanol in order to aid dispersionof the compound in a water-based solution. Panelists rate the five basictastes (sweet, salty, sour, bitter, and umami) as well as off-tastes(such as chemical, metallic, sulfur) on the LMS. Samples are served in10 ml portions at room temperature. The purpose of the test is todetermine the highest concentration at which there is no objectionableoff-taste, and determine if obvious enhancement of sweet taste exists atany of the concentrations tested.

If the compound is effective and does not have objectionable off-tastes,it is tested with a trained (expert panel) in a larger study.

For example: Five panelists evaluated 1, 3, 10, 30, and 100 uM XVI-3 inwater and in 4% sucrose solution. All samples with compound werebalanced for ethanol at 0.1% (aids in dispersion of compound). Panelistswere asked to rate basic tastes and off-tastes using the LMS for eachsample tasted. When panelists noted sweetness in any sample, they wereasked to taste reference samples of sucrose (2, 4, 6, 8% sucrose) toestimate equivalent sweetness.

A trained (expert) panel was used to further evaluate compounds that hadbeen tested with the preliminary taste test.

Panelists for the trained panel were selected from the larger group ofqualifying taste panelists. Panelists were further trained on sweettaste by ranking and rating experiments using sucrose solutions.Panelists completed a series of ranking, rating, and difference fromreference tests with sweet solutions. In ranking and rating experiments,panelists evaluated sucrose concentrations (2, 4, 6, 8% (w/v)) sucrose.

Compounds tested by the trained panel were evaluated in difference fromreference experiments. Panelists were given reference samples of variousconcentrations (2,4,6, or 8% (w/v) sucrose) and asked to rate samples ona scale of −5 to +5 in terms of difference in sweet taste from thereference (score: −5=much less sweet taste than the reference; 0=samesweet taste as the reference; +5=much more sweet taste than thereference). Test samples were solutions with varying amounts of sucroseand compound. Typically, each session compared the reference sample(labeled as REF) to numerous test samples (labeled with 3-digit blindingcodes). Tests typically included various samples with varyingconcentrations of sucrose, as well as one blind sample of the referenceitself, to evaluate panel accuracy. Compounds were tested against thereference in samples with and without 4% or 6% sucrose. All samples werepresented in 10 ml volumes at room temperature. Furthermore, todetermine the sweetness of the compound alone, a reference solution wasprepared at the designated concentration and compared to the thresholdsweetness of sucrose (2%).

Example 20 HTS Protocol for Umami Tastants

HEK-G_(α15) cells were engineered to inducibly express T1R1/T1R3 usingthe GeneSwitch system (Invitrogen). pGene-derived zeocin-resistantexpression vectors for human T1R1 and T1R3 (plasmid SXV603 for T1R1 andSXV611 for T1R3) and a puromycin-resistant pSwitch-derived vector thatcarries the GeneSwitch protein (plasmid SXV628) were linearized andcotransfected into the HEK-G_(α15) cell line. Zeocin-resistant andpuromycin-resistant colonies were selected, expanded, induced withvariable amounts of mifepristone, and tested by calcium imaging forresponses to umami taste stimuli. Cells were selected in 0.5 μg/mlpuromycin (CAL BIOCHEM) and 100 μg/ml Zeocin (Invitrogen) at 37° C. inDulbecco's Modified Eagle Medium supplemented with GlutaMAX, (10%dialyzed FBS, and 3 ug/ml blasticidin. Resistant colonies were expanded,and their responses to umami taste stimuli following induction with10⁻¹⁰ M mifepristone determined by fluorescence microscopy following themethods of Li, et al., PNAS (2002) 99(7):4692-4696. For automatedfluorometric imaging on FLIPR instrumentation (Molecular Device), cellsfrom one clone (designated clone I-17) were seeded into 96- or 384-wellplates (approximately 80,000 cell per well) in the presence of 10⁻¹⁰ Mmifepristone and incubated for 48 hours. Cells were then loaded with thecalcium dye fluo-4-AM (Molecular Probes), 3 μM in PBS, for 1.5 hours atroom temperature. After replacement with 50 μl PBS, stimulation wasperformed at room temperature by the addition of 50 μl PBS supplementedwith different stimuli. The maximal fluorescence from four independentdeterminations were averaged, corrected for background fluorescencemeasured prior to compound addition, and normalized to the response to0.002 mM ionomycin (CALBIOCHEM).

Example 21 Taste Test Protocol for Umami Tastants

Basic Training of Sensory Tasters: Tasters were trained to evaluate thetaste of aqueous solutions (5 mL each, “swash and spit”) of thefollowing standard taste compounds by using the triangle test asdescribed in the literature: sucrose (50 mM) for sweet taste; citricacid (5 mM) or lactic acid (20 mM) for sour taste; NaCl (12 mM) forsalty taste, quinine (10 μM) or caffeine (1 mM) for bitter taste; andmonosodium glutamate (8 mM) for umami or “savory” taste.

Training for Umami Taste: Tasters were given 1-3 sets of 6 MSG and/orMSG-IMP samples ranging from 3-60 mM MSG and 0-200 μM IMP, each arrangedin the tray in ascending concentration. This exercise gave the subjectpractice doing dose response evaluations. Then another set was made upof the same six samples, but were given in random order. The subject wasthen asked to arrange the samples in ascending intensity and then torate their umami intensity.

Qualifying Taste Panelists: Tasters were subjected to a standard twoalternative forced choice (2AFC) test with 5 pairs of taste samples.They were asked to make a choice of the most umami sample from twosamples (a pair). The test contains two easy pairs, two with mediumdifficulty, and one difficult pair. Tasters who could differentiate themedium difficulty pairs were selected as panelists.

Pilot/Qualitative taste test of Umami Enhancer Candidate (UEC) by asmall group of panelists: Taste samples of appropriate concentrations(usually 1-50 μM) were made in water (use minimum amount of ethanol ifnot soluble); Taste UEC alone at 30 and/or 50 μM for umami and otherattributes. Rate those taste attributes on the appropriate LabeledMagnitude Scale (LMS) on the screening ballot; if UEC has no/low umamiand other tastes, then move forward to discrimination test; comparecertain concentration of MSG, e.g., 12 mM and 12 mM MSG+30 μM UEC todetermine if there is any enhancement; rate the perceived umamiintensity on the appropriate LMS on the screening ballot; varyconcentration of UEC and/or MSG to find the best combination; decidewhat solutions to use in panel screening; record all procedures and dataincluding description of study, sample prep, sample arrangement, ballotsand sign up sheet for panelists, data entry and evaluation.

2AFC Panel Screening of UEC: Run panel screening with qualifiedpanelists using protocols generated from the pilot tasting; record allprocedures and data; prepare summary report with statisticallysignificant conclusions, if any.

Example 22 Quantitative Taste Tests for Compounds 2725761 and 3756807

Quantitative taste tests for compounds 2725761 and 3756807 were runaccording to procedures presented above. It was found that both of themhave some enhancement for MSG, in addition to their additive effect ofthe umami intensity.

Example 23 Synthesis of Compounds 2725761 and 3756807

Compounds 2725761 and 3756807 are prepared as shown in Example 22, fromtheir corresponding acids and amines. The products are purified byconventional methods, e.g., basic and acidic aqueous washes, orpreparative HPLC. The structures of those compounds were confirmed basedon usual analytical methods, e.g., NMR and LCMS. This method can also beused to synthesize any of the compounds found in Tables 1-5.

Example 24 Cell Based Assays

Cells were grown and maintained at 37° C. in Dulbecco's Modified EagleMedium (DMEM) supplemented with 10% FBS and MEM non-essential aminoacids (Gibco BRL); media for G_(α15) cells also contained 3 μg ml⁻¹blasticidin (Gibco BRL). For calcium-imaging experiments, cells werefirst seeded onto 48-well tissue-culture plates (approximately 30,000cells per well), and transfected using Mirus TransIt-293 (PanVera).Transfection efficiencies, which were estimated by cotransfection withan RFP expression vector, were typically approximately 60%. To minimizeglutamate-induced and glucose-induced desensitization, supplemented DMEMwas replaced with low-glucose DMEM supplemented with GlutaMAX and 10%dialyzed FBS (Gibco BRL) approximately 24 hours after transfection.After an additional 24 hours, cells were loaded with the calcium dyefluo-4-AM (Molecular Probes), 3 μM in Dulbecco's PBS buffer (DPBS,GibcoBRL), for 1.5 hours at room temperature. After replacement with 100μl DPBS, stimulation was performed at room temperature by addition of100 μl DPBS supplemented with taste stimuli. Calcium mobilization wasmonitored on an Axiovert S100 microscope equipped with an inverted10×/0.5 LWD piano fluor objective (Zeiss) and a cooled CCD camera(Princeton Instruments). Fluorescence images were acquired at 480 nmexcitation and 535 nm emission, and analyzed with Imaging Workbench 4.0software (Axon Instruments). T1R receptor activity was quantitated bycounting the number of responding cells 30 seconds after stimulusaddition.

1. A heteromeric T1R2/T1R3 receptor that responds to sweet or umamitaste compounds comprising, a T1R2 subunit and a T1R3 subunit whereinthe T1R2 subunit contains an extracellular region that possesses atleast 90% sequence identity to the extracellular region of the humanT1R2 polypeptide in SEQ ID NO:16 and a transmembrane and intracellularregion that respectively possess at least 90% sequence identity to thetransmembrane and intracellular region of the rat T1R2 polypeptide inSEQ ID NO:17 and the T1R3 subunit is a polypeptide that possesses atleast 90% sequence identity to the rat T1R3 subunit contained in SEQ IDNO:18 or is a T1R3 polypeptide containing an extracellular region thatpossesses at least 90% sequence identity to the rat T1R3 extracellularregion in SEQ ID NO:18 and containing transmembrane and intracellularregions that respectively possess at least 90% sequence identity to thehuman transmembrane and intracellular regions in SEQ ID NO:19 orcomprises a T1R3 polypeptide that possesses at least 90% sequenceidentity to the rat T1R3 subunit contained in SEQ ID NO:18.
 2. Theheteromeric T1R2/T1R3 taste receptor of claim 1 that responds to sweetor umami taste compounds comprising, a T1R2 subunit and a T1R3 subunit,wherein the T1R2 subunit contains an extracellular region that possessesat least 90% sequence identity to the extracellular region of the humanT1R2 polypeptide in SEQ ID NO:16 and a transmembrane and intracellularregion that possesses at least 90% sequence identity to thetransmembrane and intracellular region of the rat T1R2 polypeptide inSEQ ID NO:17 and the T1R3 subunit is a T1R3 polypeptide containing theT1R2 extracellular region in SEQ ID NO:18 and the human transmembraneand intracellular regions in SEQ ID NO:19.
 3. The heteromeric T1R2/T1R3taste receptor of claim 2 wherein the T1R2 subunit comprises a humanextracellular domain, and a rat transmembrane domain and a ratintracellular domain having the sequences which are respectively in SEQID NO:16 and
 17. 4. The heteromeric T1R2/T1R3 taste receptor of claim 2wherein the T1R2 subunit comprises a rat extracellular domain, a humantransmembrane domain and a human intracellular domain. having thesequences which are respectively are in SEQ ID NO:18 and 19.